Could you insure yourself either given the fact that- you bought the
property on mortgage?
A mortgage does not generally have the legal effect of insurance. You owe the debt to the lender, which is a strict liability obligation, without regard to the value of the collateral. So, if the collateral is devalued, you still owe the debt, whether or not the lender forecloses. For example, if you sell the property for less than the amount of the debt (i.e. a short sale), you remain personally liable for the deficiency between the proceeds of the sale, and the amount owed on the debt, unless the lender consents.
The exception to this is a "non-recourse mortgage", which is a mortgage in which the lender expressly agrees to accept the collateral as the sole means from which the debt can be repaid in a compulsory fashion. The financial crisis of 2009 was mostly a function of a handful U.S. states like California and Florida with economically important mortgage markets having residential mortgage loans that were as a matter of law, non-recourse debts (or nearly non-recourse debts) compounded by people making risky decisions knowing that they didn't face the risk of a deficiency judgment.
But, non-recourse mortgage lending is very rare in the U.K. (the linked material inaccurately asserts that mortgages in "most of the United States" are non-recourse, however, when, in fact, that is true only in a small minority of U.S. states, probably not more than five state in all, although a couple of the states that are close to non-recourse for residential mortgages, like Florida and California, are economically important U.S. mortgage markets).
Of course, even in the case of a debtor who is obligated on a full recourse mortgage, it may be in the interest of the lender to write off the deficiency judgment, rather than seeking to recovery the debt from the borrower, if the borrower is apart from the collateral, judgment-proof or very nearly so. The lender is taking the risk that both the collateral will be insufficient and that the borrower will also be unable to pay the debt, and when that happens the lender takes a loss.
Or is there any insurance which could protect you against massive
downfall in any Unforseen compulsory purchase situation.
In the United States, lenders often buy what is called "mortgage insurance" at the expense of the buyer, when the down payment on the property is under 20% of the purchase price, that remains in force until 20% of the purchase price has been paid through a combination of a down payment and principal payments on the loan. But, this form of insurance is not generally available to property owners themselves as a matter of economic reality, possibly because it is an uninsurable risk, and possibly because there isn't enough economic demand for it at prices that would make it profitable to do so.
Also, when a lender receives a payment on a mortgage insurance policy, the insurance company receives in exchange, all of the rights that the lender had to sue the borrower for a deficiency judgment, if any. These are called the insurance company's "subrogation rights."
I have certainly also never seen any form of insurance for an unfavorably bad outcome in an eminent domain/condemnation proceeding for any reason. In a case like that the court determines as a matter of law (in one of the few proceedings where there is still a right to a jury in the U.K.) what the fair market value of the property is at the time of the condemnation, and that determination would complicate recovery on any insurance policy because you would need to have a proof of the loss.
Generally speaking, a mere decline in the fair market value of real estate, in and of itself, it not considered to be an insurable loss, of the owner of the real estate, for insurance law purposes.
A very close cousin of insurance contracts are bonding contracts. When a third party is unsure that you will be able to perform a contract or pay a debt, you can encourage them to do business with you by having a bonding company agree to meet your obligations up to a certain dollar amount if you are unable to do so, usually, in exchange for a fee, a right to sue you if they have to make a payment for the amount that they had to pay, and sometimes for some sort of collateral to protect them against the risk that they are taking.
But, bonding companies don't generally provide bonds for residential mortgage customers at any price, because someone who needs a bond on a debt like that is unlikely to be able to ever repay the bonding company for its loss if it does have to pay the mortgage debt, and because bonding companies would need to set aside too much money as financial reserves against this risk to be prepared in the event that it had to pay a lot of claims due to collapsing real estate price bubble or something like that.
Or is there any way to mitigate this risk / avoid this risk?
Option To Sell Contracts
On the buyer's side the primary "insurance-like" legal instrument would be for the buyer to purchase from a third-party a legal option to sell the property at a specific price that is lower than the current purchase price.
Such an option would probably be legal to enter into, and, with the proper regulatory compliance and financial disclosures, a firm could sell such options to residential property buyers. But, as a matter of practical economic reality, I have never encountered a transaction in which someone actually did that with an unrelated third-party in an arms-length business transaction.
In the financial crisis of 2009, secondary market mortgage debt buyers had purchased options to hedge against just this kind of risk, but the counterparties who were obligated to cover the losses pursuant to those options didn't have sufficient reserves and other assets to cover the losses that they were obligated to pay, and so the people who had purchased these options were stiffed anyway. This is because prominent credit rating firms for businesses (of which there were only three or four in the United States) failed to properly evaluate the fact that the risk of one claim under this kind of option was not independent of the risk of other claims under similar options happening at the same time, and in general, failed to accurately evaluate the risk of counterparties being unable to perform their sides of the contracts because the counterparties were be prominent financial companies that had never failed before. But, faced with this situation, almost every investment bank in the United States either went bankrupt or was acquired by another financial company that was not allowed to engage in this kind of derivatives transaction.
Mortgage insurance companies, in contrast, paid all of the claims against them, because state insurance regulators had adequately evaluated the risks and forced the mortgage insurance companies to set aside adequate reserves to pay claims in the event of a situation like the 2009 financial crisis. But, because the need to set aside reserves made mortgage insurance (which also had premiums that were not tax deductible to the property owner) made mortgage insurance more expensive than having the same bank give someone both a first mortgage and a higher interest second mortgage on a residence, and then entering option contracts to control their risk of loss in the event of a real estate devaluation that made the second mortgage uncollectible, mortgage insurance companies had a pretty low market share of the financial services providers who were addressing the property devaluation risk for mortgage lenders.
Long Term Leases In Lieu Of Purchasing With A Mortgage
Another alternative would be to enter into a favorable long term lease of the property, which would be owned by somebody else, rather than actually buying it. If the lease had a term automatically terminating upon a compulsory purchase such as an eminent domain proceeding, the landlord and not the tenant, would bear the risk of loss in the event that the property bad massively devalued due to a change in prevailing market prices (although the landlord would also benefit in the event of massive appreciation at the termination of the lease, although that might be long in the future). This would be an extremely uncommon arrangement for someone to make with their own residence, but isn't unthinkable.
For example, over in Ireland, the Guinness Brewing Corporation rents rather than owns most of the real estate that it used on long term 999 year leases (if I recall correctly), possibly out of concern for this possibility, which was a very real one at the time that those leases were put in place.
Similarly, my childhood home in a university town was built on land leased from the university on a 99 year lease in the 1960s or 1970s, that was later converted to absolute ownership of the land by my faculty and administrator parents about fifty years into the original lease by mutual agreement between the university and the original home builder. But, that wouldn't really be strictly analogous, because the house was purchased by my parents subject to a full recourse mortgage secured by the building and their rights as tenants on the land lease, so they weren't really protected from a mass devaluation of the property. The transaction didn't really hedge against an economic downturn. Instead, it effectively gave the university the right to buy back the land at the cost of compensating my parents for the value of the residence built on that land, if it wanted to expand.
But, it would still probably be possible and legal to enter into a long term lease whose terms did hedge against that possibility, possibly with an option to buy the premises after a certain number of years long in the future when devaluation relative to the purchase price was much less likely to due gradual annual appreciation and inflation over that time period (e.g. 40 years out), if you could find someone willing to serve as a landlord in that kind of deal.
Caveat Regarding Taxation
The way business transactions are structured is frequently heavily driven not just by the underlying economic effect of the transactions, but also by the tax implications of the transactions. But, I am not familiar enough with the tax laws of the United Kingdom to evaluate that piece of the puzzle.
In the context of the question, the most viable alternative to limit risk downside devaluation risk would be to enter into a long term lease rather than buying the residence. But, that only makes sense if there are not big tax benefits to owning a residence with a mortgage as opposed to leasing one. In the United States, there are huge tax incentives to buy rather than lease. But, I don't know if there are similar tax incentives in the United Kingdom that might make a long term lease solution less attractive.
For example, a large share of all businesses in the United States lease rather than own the real property that they use, and a large share of skyscrapers and other high rise buildings in the United States are built on leased land. But, those transactions are structured as long term leases, rather than purchase transactions, primarily for tax reasons.
Under U.S. tax law, businesses can't treat money spent to purchase raw land or principal payments on mortgages as an expense for tax purposes, but can treat the full amount of any lease payment the business pays to a landlord as an expense for tax purposes. So, transactions are structured accordingly.
For example, in a high rise transaction, the building owner pays the fully deductible long term land lease payment to the owner of the land (usually a non-profit that isn't worried about having taxable income not matched by an expense deduction), while the building owner can make up for not being able to deduct principal payment expense on the construction loan by being able to take depreciation deductions on the building itself as an expense in a similar amount. The non-profits usually don't borrow the money to buy the land that they lease to high rise owners. Instead, this is an investment option for cash rich, stable non-profits that need to find a way to get reliable, low risk, long term passively managed returns on their investments. And, the risk of devaluation is much lower for a long term investor with a forty year time horizon than it is for property owner with a shorter time horizon.
But, without these tax incentives, there would be far less real estate leasing by businesses in the United States, and a desire to hedge against significant real estate devaluations is a far more secondary reason for businesses to lease of real estate in the United States.