I'm halfway though a similar project in SE London, I think your best bet is to get clear plans (couldn't recommend the architect I used.. though got planning okay) and a clear list of exactly what work you want done, then get a number of builders (who will manage the trades) to quote. The builder I am using, (who did a loft conversion for a friend) used the structural engineer Porthousedean, I was impressed with the engineers thoroughness/price, they will need accurate plans to go off. I don't think it will really cost you any more to have a builder manage it and they can then co-ordinate the work much better.
I'm halfway though a similar project in SE London, I think your best bet is to get clear plans (couldn't recommend the architect I used.. though got planning okay) and a clear list of exactly what work you want done, then get a number of builders (who will manage the trades) to quote. The builder I am using, (who did a loft conversion for a friend) used the structural engineer Porthousedean, I was impressed with the engineers thoroughness/price, they will need accurate plans to go off. I don't think it will really cost you any more to have a builder manage it and they can then co-ordinate the work much better.