and don't want to be rediculed/intimidated by a bunch of
civil servants until absolutely necessary.

Building Control Officers are usually very approachable, particlarly for
queries like your's. They'll let you know what they consider reasonable for
the wall construction. I'd guess 25mm of plasterboard on a stud wall would
be fine, or you could use light concrete block. Speak to them, though, as
they might have different ideas.

Any doors will have to be fire doors with intumescent strips. There are lots
of other issues that need to be addressed. For a start, you'll need to bring
insulation up to date, including the floor, if you want the rooms created to
be considered habitable.

Christian.