Have you considered a complete room-in-the-roof system? They can be fully designed to meet your needs, factory engineered to form a complete habitable roofing solution.
Roofing solutions can save a great deal of time on site - and can be fully installed in less than a day. They are designed to meet the highest fire performance and are fully insulated with CE marked products in controlled manufacturing conditions to ensure a high performing, warm building envelope.
Roofspace Solutions - based in the Midlands offer UK-wide solutions and are a Saint-Gobain brand.