I have repeatedly bought properties without building regulations. If the are the right one.
However you do have to have them to resell.
I'd be asking the building officer at the council, questions, he should be able to refer to his notes. He will know exactly what he needs to see to sign it off.
This will give you an idea of how much work needs undoing and redoing. It's not a case of him just signing it off as regulations change and it now has to conform with the latest specifications. Also things get boxed in and plastered.
Electrics come under different regulations and although are signed off as a whole by building regs they are registered first by the company who installs them and signs them off.
With this information I'd write a carefully constructed letter with this to offer a balanced reduced offer based on the work that needs doing to make it legal.
Remember the costs may not be straightforward, taking out to access and the redo can be more work than initially doing the work n the first place.
You may decide to carry the costs yourselves if this might need changing in the future. This is what I did with a badly converted bungalow and very illegal stairs.
We knew we would knock the inside of the place around and relocate the stairs so got building regulations fir the new stairs.