People look for different things from schools. My view was almost focused on academics, so here are some thoughts. Unfortunately, in this area best schools (by results) are single sex.
Thomas's is a good and happy school. Not terribly academic, but big on confidence, drama, art, etc. They don't prepare boys for 7/8+ but, as far as I understand, do prep girls for 11+.
Don't know enough about St.Nicolas. It's not a terribly academic school but does have an option of French curriculum. Not sure what their exits are like.
The Vale is co-ed on paper, but many classess have very small number of girls. Drawback is that it finishes at 8, so kids do have to take the exams to get into other schools. As far as I understand, school does well academically. Facilities are not great - no outdoor space.
GEMs - seems to me to be at the same league with St.Nicolas.
Finally, don't know if you have a nursery lined up - heard great things from a number of friends about Iverna Gardens Montessori.
You should also look at the Good schools guide for more info.