Who Owns The Bunker?
Your relative obviously owns the cellar up to their land property boundary.
The people who own the other land would have originally been the owners of that portion of the bunker, assuming that it was not permissive (if it was permissive, an easement by estoppel probably would have arisen). So, then the question is, whether the people in the chain of title to the relative acquired ownership of that portion of the bunker by adverse possession.
Normally, adverse possession is acquired when someone occupies land under a claim of right that is open, notorious and hostile. While the use of the bunker would be "hostile" in this case (because the other property owners didn't have access to the bunker), it is not at all clear that it was "open and notorious". Indeed, apparently, the other land owners weren't aware that it existed. So, there would probably not be "squatter's rights" in this situation.
Also, adverse possession can't run against the sovereign, so to the extent that the bunker goes under public land (e.g. a public street), that also can't be adversely possessed and that portion of the bunker remains the property of the sovereign land owner.
@PaulJohnson in a comment to another post notes that:
It sounds like your relative has one of the secret bunkers built for
the "Auxiliary Units" who were to wage guerilla war from behind the
lines after a successful German invasion.
If this were the case, and the government authorized its construction, there would probably be an implied easement that would allow the bunker to be the property of the person owning the entry to it, rather than the other property owners. But, as you note, proving that case would be difficult. It might be possible to scour declassified civil defense records from the WWII era to determine if this was the case, but you might need to employ an archivists or historian to get to the bottom of this question.
What if they sell it?
Suppose that they do sell it. What happens?
Since title is certificated in Britain, there would be no title insurance company to compensate the buyer if someone later claimed to own the property. Your relative would have a warranty of title inherent in the deed to the portion of the property that is legally described in the deed (unless the property were sold by a quitclaim deed specifically disavowing any promise that what was sold was owned by the seller), but that warranty would probably not include the portions of the bunker outside the boundaries of their lot because that is not included in the legal description of the property on the property certificate or the deed.
So, if it was sold, the buyer might not have any claim against the seller if the buyer did not get good title to the entire bunker, and would have no one to sue at all, if your relatives died before litigation over ownership of the bunker arose.
The buyer might sue your relatives for common law fraud if the bunker were described in the marketing materials for the sale, but if they were told that some of the bunker went outside the lot and that its legal status was unknown, or if the property was sold by a quitclaim deed, that suit would probably not be successful.
Depending upon whether the owner is on good terms with his neighbors, the owner could probably buy the subsurface rights or some sort of easement to that property from the neighbors under whose land it runs, and might even be able to purchase such rights from the local council where it runs under a street.
Negotiating the price would be tricky. On one hand, it doesn't hurt the other owners at all. On the other hand, they have the power to deprive your relatives of all use of the property. Often deals like this are done for nominal consideration between people on good terms with each other, but for extortionate prices when people are not on good terms with each other.