I know you've already received some recommendations on loft building companies but thought I'd give you another choice. It's always nice to have a few companies so that you can compare.
I've used New London Interiors twice now - once for a total refurb of our old place in Clapham and the second time round for a large two bedroom loft conversion in our new place in Colliers Wood. Adam and his team are super efficient and he has great insight and was very helpful with recommendations on sourcing doors, fittings etc which we had real specific ideas on. Very neat work, delivered on time and on budget.
I would defnitely recommend you have a chat with him.
New London Interiors
07950 136 554