We have been happy with Clapham Park Group Practice, although we have been lucky enough not to need them for 2-3 years now
When I was pregnant they recommended St Thomas' as they did shared care with the practice and I had most of my visits (slightly complex situation that required closer monitoring) at the surgery, at the midwives' centre that used to be at the back of the same building or at Clapham Manor Centre.
We were very happy with St Thomas' (8 years ago!): they took all the time needed to explain things, and despite being swept off their feet the staff were kind, helpful and never brushed off any concerns.
I don't think you can get it wrong with any of the teaching hospitals near here, so I would take into account the logistics of appointments (when and where) and check how that would fit with your day logistics - commute into work, care of other children, etc.
Good luck for a smooth pregnancy!