Taller is advertising based and definitely a hagiography.
GSG has a much larger team of consultants and has 'mystery shoppers' as well as declared visits to schools. It is much more independent (as it needs a good reputation for accuracy ) has been going for years, and offers both the directory of published advice, and individual consulting service. It is not a placement service. If you want a selective school, you have to apply at the normal time, or o throw the usual assessments for ad hoc places. Info on how to do that is also readily available directly from the schools.
For the published directories of schools (whoever the publisher) you need to ad lots of the reviews and learn the 'code' for example, one that is all about the headmaster, means the school is all about the head and if you do not take to him then the school will be a poor fit for your family (no matter how much those who like that style are happy there)
The other big name in consulting about school places is Gabbitas, who offer a very good service. Wouid you get their fee covered as part of a relocation package?
And of course, if you know which part of London you want to live in, and have a short list of schools you think you like the look of, you couild try a MN thread on them