As I see it, the issue is not really about rules and regulations, but about whether something might actually fall down or be dangerous! Building Regs exist to check for that kind of thing - e.g. have important structural changes been made, is the wiring safe, will the floor of your loft hold your weight, can you get out quickly enough if there's a fire...?
Most conservatories are exempt - i.e. they don't need regs. But extensions that are 'non-standard' or possibly risky do need BR - e.g. if they are over a certain size, involved knocking down bits of walls, aren't built on level/flat ground, are built over sewers, etc.
The indemnity policies you can buy only insure you against the council forcing you to put right something that doesn't meet BR requirements. They do not insure you against anything falling down or going wrong in other ways. That's why you need a good surveyor to tell you whether the work is sound.
The surveyor who's already inspected may not have looked at everything necessary to determine whether the structure is safe. For instance, if s/he assumed the extension did have BR, s/he would expect all the works to have been inspected - so s/he may not look at the foundations, for instance. But if s/he is now told BR was not obtained, and therefore that no-one has inspected the foundations to check they are strong/deep enough, s/he will probably want to dig a hole and inspect them her/himself.
Incidentally - and it is a very BIG incidentally IMO - my solicitor's advice is that normal household buildings insurance almost certainly does not cover work that should have had BR but doesn't. Therefore, if your non-approved extension falls down, or you fall through the floor of the attic bedroom that was never signed off, or your house falls down because a structural wall was removed without meeting BR, then your insurance will not pay for it.
If I were you, there is no way I would buy a house under these circumstances. IMO, you must get a surveyor to check the structure of the extension and the bits where the walls are missing, and tell you what would need to be done (if anything) to make everything safe.