Well OP here are the top 10 girls' secondary schools in the country from the most recent league tables:
Wycombe Abbey School
The Haberdashers' Aske's School for Girls
North London Collegiate School
Guildford High School for Girls
Oxford High School GDST
The Lady Eleanor Holles School
Withington Girls' School
City of London School For Girls
South Hampstead High School GDST
James Allen's Girls' School
My DDs go to the junior school of one of those on the list and to me, one of the BIG selling points is the more-or-less automatic transfer to the senior school in Year 7 (barring any problems or a lack of fit, in which case I would immediately start looking for a school that fit better!) I believe the 11+ and Common Entrance process is very stressful for girls and their families and I am relieved that we won't (hopefully) have to go through it. (As an aside, I also work in an independent London prep school that does not have a senior school, and I see all the exam prep and stress that our Year 6's go through every year).
I am ambitious for my DDs, they are bright and I don't apologise for that. I want them to have any door open to them that they would choose, and to me starting an education at a top school can only help with that - as long as they are happy of course.
I was quite surprised at how "gentle" the junior school actually is. I don't feel there is a lot of pressure, the girls and the parents are absolutely lovely, the teachers are caring and involved, the Headmistress is fantastic.
OP from what you have said, if I were you I would seriously consider sending your DD to the Habs junior school (another one here who has never heard it called Aske's)
Good luck with it - I found this whole schooling / education thing a very stressful part of being a parent.
Sorry for the epic post!