So basically get insured initially with the council signed off house (all fully compliant), make the changes, tell them to come back to inspect. Worst case scenario they say not happy, close it off again so either a) look for an insurance co who'll accept it or b) close it.
OR they might say, spot on no bother bosco, pay me extra for the square footage.
Do people generally tell their insurance company that they've conerted the attic and that it may not comply with regs ... or are loads of people unwittingly at risk. I presume the fact that you didnt realise it didnt meet the regs is no defence.