Since frogs mentioned me ...
I have mixed views on A'corn. We began there with ds in reception, dd in nursery. The nursery was great - dd loved it and, in particular, I very much liked the other parents - all kinds of nationalities, but all quite consistently well-educated & cosmopolitan: nice group of people to know. (A'corn users are often professional expats, in London for a few years' posting: but - NB - as a result the demographic is transient. Children have to get used to losing a friend or two each term.) Dd was happy there & would I think have continued perfectly happy through the school.
We left though because ds, who is an educational square peg (bright with social difficulties), couldn't really be catered for. Individual teachers did try hard, but he was clearly off their scale of expertise, & I got weary of being called in & asked, slightly unsympathetically, to "explain" him (when indeed I couldn't, as his social difficulties mainly show up in school situations so weren't a side of him I knew then).
Bear in mind also that the St John's Wood bit only goes up to year 1 - from year 2 on they are down on the Marylebone Road, & from year 4? further south yet - so the initial convenience doesn't play on. However, if you're just looking for nursery/reception, it is good for that - especially nursery.