Thomas's has a LOT of Oxbridge-educated parents so Paddy, Sofia and Papinian would all feel right at home there.
I'm a bit concerned that most of the people commenting on here aren't actually Thomas's parents. I am and in all my years there I've never heard of a child being 'managed out'. However, you have to bear in mind that while it caters for a broad range of abilities, it's a fast-paced school, with long hours (albeit long holidays) and lots going on, so it doesn't suit every child, and people do move their kids if they either feel they're struggling or if they think another school is a better fit (e.g. I've come across children being moved because the boys' sports are better at other schools). All those Oxbridge parents tend to have quite bright offspring so the average ability is pretty high. Also don't forget that it is quite easy to move kids around after Reception so if the school doesn't suit it's not the end of the world.
If you want to get a better feel for the school, they're having a Xmas Fair on Saturday. I'm sure they'd welcome any NappyValleyNetters wanting to have a snoop (and spend some money!). Take a note of the kids and see if you think that's the sort of child you'd like yours to grow into (although bear in mind they'll all be on a collective sugar rush from the sweet stall).