Because they are all owned by the same company, I think the quality of education will be roughly the same throughout. Which one is best will be a matter of personal consideration, as they have different features. ACS Cobham, for example, offers a US-based curriculum. Where are you originally from? Are you already in London?
Hope this helped,
Hillingdon is the closest to London, yes. It has both the IB and the US curricula. What is your child's curriculum at the moment?