The whole situation is a nightmare, nationally.
I agree with PPs that indemnity insurance is a scam (wish my solicitor had been sufficiently on the ball to realise that when I bought the old house).
Using my experience as an example, I lived in a row of mid-Victorian terrances. Probably 90% of those had loft conversions. I would guess of those, about half were probably done without buildings consent, covering a period from the 80s through 90s. No paper trails, several house moves since they'd been done. Lots of buyers taking a "well there's indemnity insurance" attitude without really understanding (or not having been properly informed by their solicitors) what that actually meant.
Result - lots of people living in houses without building regs consent without understanding the full implications (and it's not just resale that's an issue, it's things like insurance; a friend of mine got caught out when she had a house fire and her house insurance argued that because it turned out - after the fact - that she was in a house without building regs consent for the loft conversion, this nullified the insurance!
)
And lots of buyers starting to get wise to the complexities of the situation (or as in your case, reasonably enough trying to do some digging to make sure your own interests are protected, not realising the can of worms you're opening), unwilling to purchase or insisting retrospective building consent is sought (complex, because what might actually have met building regs at the time it was done, mid 90s say, almost certainly won't now - a major pitfall nowadays is getting enough insulation into the ceiling without dropping the ceiling height too low - many Victorian houses simply cannot be converted to meet modern building regs for this reason, not without dropping the level of the floor and losing some ceiling height from the room below, because the roof line typically can't be altered, specially not in a conservation area).
I suspect we're sitting on millions of older houses in this country which are currently in a grey area concerning resale and even the validity of the occupier's insurance.
As for what to do in your circs: if you've already negotiated a hefty discount, and now (inadvertently) pissed off the sellers because they now can't sell to anyone else if their sale falls through, can you afford to take the financial hit on getting the work done? Get some quotes from local builders who offer a design and build including building regs sign-off service?