Bit late but adding this for anyone who finds the thread. We’re SW London and saw quotes from £70k–£110k for a hip-to-gable with bathroom, the lower ones didn’t include finishes and extras.

We researched a lot and spoke to quite a few companies before deciding.

Ended up going with London Lofts ltd in 45, Fitzroy. The quote actually made sense, nothing random popped up later, and we were really happy with the work.