Originally Posted By: LittleBlueThing
Occasionally you get an opportunity to renovate and you may want to lose 4" off an external wall by using plasterboard over battens with well fitted insulation.

I don't know what kind of insulation is commonly used (or even available) in the UK, but if you can get it, polyisocyanurate boards would be more space-efficient than your (I assume) fiberglass suggestion.
_________________________
Bitt Faulk