In england-and-wales you have no binding power to determine what will happen to your body after your death aside from decisions involving organ donation. The decision of how to dispose of your body lies with the personal representative of your estate. If you die testate (with a will) then that is the person(s) you name as executors. If you die intestate (without a will) then it is the administrator(s) of your estate which is determined by a legal order of precedence from various relatives. A personal representative can also delegate the decision to another person by way of power of attorney albeit this can be revoked at any time.
There is a general obligation on the personal representatives to dispose of your body properly, but the details of how to do so are entirely within their discretion.
It can still be helpful (persuasive) to express your wish in the will. While not binding, it can be useful in the event that there is a dispute over what to do with your body (e.g. a disagreement between different executors, or a lawsuit from a beneficiary who wants to limit the amount of money spent on the funeral). Your express wishes may be a factor which would influence a court's decision where multiple alternative outcomes are possible.
Appoint a co-operative executor
The best plan of action is therefore to write a will and appoint executors whom you believe will be the most likely to respect your wishes. This could be a paid professional (e.g. a lawyer) if you cannot find such a person among your friends and family. The benefit of a professional is that you can remove personal feelings from the equation; arranging the funeral according to your wishes will simply be a matter of them doing the job they were paid for. The downside is that this may get expensive depending on how much work is involved.
In my work, I sometimes deal with cases where people wish to control what happens to their body after death. We adopt an alternative strategy, but it is important to note that this is not free of issues and has not been tested in the courts.
A basic principle of a will is that the executors have a duty to act in the best interests of the beneficiaries (once all debts etc. have first been paid out of the estate). If your estate will have net assets then you can create a conditional legacy along the lines of; "Person X is to receive asset Y on condition that after my death my funeral is conducted as follows: [...] and otherwise asset Y is to be treated as bona vacantia".
This creates a duty for the executor to act in the best interests of person X; in other words to make sure the funeral is conducted in line with your wishes so that person X can receive asset Y. Person X will have a cause of action which they can take to the courts in the event that the executor fails to do so. It's therefore important to make sure person X is aware of the provision and will be active in asserting their rights (e.g. by quickly seeking an emergency injunction). Who you choose and the amount you leave to them will of course be factors in how motivated they are.
The reason for the bona vacantia provision is to avoid creating an otherwise inevitable scenario where a different person, Z, is entitled to asset Y in the event that the funeral is not conducted as specified. This would be problematic because the executor's duties would be split between two people. Bona vacantia assets go to the Crown which is not known for actively intervening in such matters generally (e.g. in this case, taking legal action against the executor to prevent the funeral from going ahead as planned).
The problems are that (a) it is a novel approach which is untested (to my knowledge) in the courts, (b) it relies on someone asserting their rights against the executor and if you had such a person you probably could have just named them as executor in the first place, (c) your assets may not end up being inherited as you intended.