St James parent here; DD is in year 7. So it's been less than a year but so far our experience is very positive.
Specifically about your concern in the first post -
DD too was the only one from her school going there, and we had similar initial concerns about whether she would be able to make friends as she is quite shy. Thankfully no friendship issues and DD has made a good stable group of friends now, both from the junior school and the new girls. As far as I am aware there is no clique (at least so far!) and the girls mix well. The school makes an effort to ensure this happens - an early start of term day trip to Oxford for a team building exercise helped break the ice. Various group activities (cookery, drama, classes etc), a plethora of clubs (nearly 50 or so) makes sure the girls find other with similar interests.
On the academic side, from a parent's perspective, I like what the school is doing -
They have a superb humanities and mathematics department with some very experienced and skilled staff. The custom history curriculum and teaching is amazing. They do frequent assessments (test almost every two week or so), track and intervene quickly if needed. For example, following a test, some girls maybe invited for subject clinics, and/or will retake tests following some work. Homework is about an hour each weekday, and bit more in the weekend. High teacher to student ratio (better than 1:7) certainly helps with extra help with clinics etc.
I wouldn't say it's very competitive (it wouldn't have been suitable for my DD if it was), but there is some academic pressure and stretch. First term starts easy, but homework, exam etc ramps up from the middle of first term.
Also, very lively discussions in assembly and philosophy classes on a variety of subjects outside the curriculum (Iran protests, Thatcher and the unions, analysis of painting, how to handle sadness). Probably these are perfectly normal in other London indies too (I have no prior experience), but I am pleasntly surprised watching DD gradually developing a well rounded personality, lot more aware of the world around her.
Among the unusals in St James - quiet time/meditation and vegetarian food; but you are probably already aware of these?
things I don't particularly like -
The headteacher is retiring. She was one of the reasons we chose the school as we really liked her approach. The new headteacher seems nice, but it's still a change we didn't expect.
The school is a bit more religious than I imagined it would be. Not overly so (it's not a faith school), but there are regular prayers etc in assembly and we aren't a religous family.
There had been a few communication hiccups in the early months, nothing major, but things like sports fixture mixups, vaccination date missed ( corrected later), etc. Not really a biggie for us (and things have improved since), but some parents were upset. (In all such cases, reaching out to the school fixed the problem).