I guess it depends on what your medium to long term plans are. If you planning on going back to North America then an International School may be better. Another option if you arent sure where you will be is you can let your daughter finish her primary education under the UK system. I assume in september she will be in Year 5 is she turns 10 end of this year, then consider a seconadry school which offer IB at A level as that is more portable. They will still do GCSE but the curriculum will be geared towards IB.
I dont know much about London schools but some of the highly academic ones I have heard of are Bute House, St Christopher and some ones that have senior schools like City of London Girls and HABS. These schools are extremely competetive to get in and getting in half way means someone needs to have left and also depends on what position you will be on the waiting list. Others not so academic but still worth a try are Falcons Girls and Francis Holland. I am sure Londoners may give you details of some more schools. To get a general overview about the school and the inspections you can use this http://www.isi.net/
By the way if you decide on International schools in London you may find more feedback here http://talk.uk-yankee.com/
for the schools you have in mind.