Drylining the external walls up to ceiling/ roof level won't be a problem, you could use a "Gypsum" or "Kingspan" thermal laminate board which combines insulation with a plasterboard face. This product comes in various thicknesses, depending on the level of insulation you require, but a 50mm board is pretty standard. Insulating the roof void is going to prove difficult, probably impossible if you want to retain the existing timber ceilings. You have no access to the "hidden" space so how do you get insulation in there? You could "blow" in bead or fibre insulation from some openings made in the ceiling, but the insulation will migrate down towards the eaves, blocking ventilation pathways and creating further problems. I don't see any other way other than to "dryline" over the existing ceiling, fixing back to the rafters or taking the lot down and starting out with a new ceiling. The second option will at least allow you to improve ventilation (which is crtical in dormer type warm roof constructions), put in additional wiring for lighting etc. Otherwise live with spiralling heat costs. Insualting your external walls whilst neglecting the roofspace would not make economic or environmental sense. Timber sheeting is a real fire risk also and there are limitations on its use and extent specified in the Building Regulations (surface spread of flame).