"she does tend to cruise academically"
This is often as much about her peer group as well as the school. If her friends are not keen on work, it is likely that she will slide too.
Thinking just North London ....
People love King Alfred's. Don't know anyone that goes there now, so don't have a current view.
Queen's College is very nice; I think the peer group thing is important here as it is a small school, so if you have a sparky class you will do well, but if you don't it is a bit small to allow for finding another group. It does allow for individuality and so on - although they have introduced a uniform now, which maybe sends a message about how they feel about quirkiness (??). A few seem to leave looking for more academic schools.
Francis Holland Clarence Gate - clearly academic and ambitious, and the head very enthusiastic about their very wide extra-curricular provision. It is possibly a bit cramped for the number of girls there, but it certainly (if they do what they say) seem to cram a lot in. Again a few seem to leave in the first couple of years - often for boarding schools.
South Hampstead - as a larger school probably allows for much of what you want, but they are about to embark on years of building works.
Camden School for Girls - perfect if you can get in. Allows for all sorts, very individual and academically demanding.
Queen's Gate (a bit further south) - very happy, positive girls. Very positive parents. Worth a look.
If very academic and individual, St Paul's Girls' is good. However, this doesn't sound like what you are looking for.
Also always remember that for many children their personal interests are supplemented outside school - the theatrical go to drama classes, the sporty to football clubs and so on. Few schools can provide everything - however, what you do want is the school to appreciate what it is that she is interested in, and more importantly, for her to find like-minded (or just appreciative) friends.