St Margaret's is lovely. As you probably know it's a small school - very inspiring head (and nice to see a male head and reasonable proportion of male teachers in a girls only school). Very nurturing, small classes (10-12 junior, 15 senior). Lots of sport and clubs, music etc. They have "125 things to do before you leave St Margaret's" to encourage wide interests. Active PTA. They don't chase the league tables so don't get GCSE results like SHHS, NLCS, Channing etc - they are less selective than these others so I think those results reflect the input. Those in the top stream get extra lessons if needed, and, though they only go to 16, they sometimes have one or two students take an early A-level if appropriate. Girls go on to good schools post 16 - lots to Highgate, also Channing, SHHS, NLCS, Henrietta Barnett etc. I think they start the planning for post 16 schools several years before.
I suppose the only downside is if you don't like small schools - or worry that the classes are too small to make friends - and some of their facilities are off site, for example the hall in the school building is small so they use Skeel Hall for PE which isn't on their main site, but it is only 5 minutes walk away. They also use a local tennis club very close, and take a minibus to playing fields and to Copthall to swim.
If you're undecided I'm sure Mr Webster would show you round again and answer any questions you have. They are very open and welcoming to any enquiries from parents.