Hi,
We used Armstrong Simmonds Architects
www.as-architects.co.uk as our architects. I can highly recommend them. They are based in Clapham and offer a full service from concept design and planning through to appointing a contractor and over seeing the project on site. They are RIBA chartered and have a portfolio of well designed, completed projects which they can share with you, if interested.
Then we used PVAD
www.pvad.co.uk to do the building work. The whole project was pretty much seamless.
Note: Make sure that you get the plans and also structural calculations included in your price.
Hope that all helps.