For me, indemnity insurance is a red herring, and certainly not a solution. For a start, there are time limits on some aspects of building regs and planning, after which a council can't take action - so many people pay for insurance to protect themselves from a risk that has in fact passed.
But more importantly, indemnity insurance only protects you from enforcement action, not actual problems. So it will cover you if the council comes along and tells you to make changes, but it won't cover you if the extension or conversion has structural problems and cracks appear, it falls down, etc... And what's more, your 'normal' household building insurance won't cover you either, because unregulated/unapproved work is excluded.
So if you buy a house with an unregulated attic, and it turns out the floor isn't strong enough and the ceiling below collapses - you're not covered. Or if the unregulated extension has subsidence or develops cracks, or even if it collapses and damages next door as it falls - you're not covered. Or a fire starts there and spreads to the rest of the house because it has no separating doors - you're not covered. :(
I don't think vendors are always being cheeky or trying to pull a fast one - I think many people just don't realise the significance of building regs.