I'd love to know the name of the company, not Choice was it as the same has just happened to us and we're in North London too.
We have been thinking of a loft conversion for god knows how long and have been messed about by Saxon, A & W (who wrote his quote on a fag packet). Econoloft managed to write a quote (not cheap though).
It baffles me that something which seems straightforward should cost such a lot. No foundations, no cement, no bricks! We don't even want any plumbing whatsoever and the price is still extortionate.
Good luck.