You can get insulated plasterboard that adds about 25mm to the thickness of the plasterboard. if you are stripping back that much then i would recommend it – have a chat to jewsons – they will be able to advise regarding whats available – insulation isnt my main area – solar is :)
is the boiler natural gas or is it oil/LPG? are you thinking of solar in order to save money or are you doing it for the carbon reduction? if its natural gas, and you are doing it to save money – spend the money on better insulation/double glazing/more efficient boiler. if you are a tree-hugger (like me) then solar thermal is a good way to reduce your carbon footprint.
If you can let me know the postcode/direction & pitch of the roof, number of people and type of fuel you will be using, i can do a detailed analysis of what your system is likley to do for you.