Kingsdale, a state school with no catchment, runs buses and in rural areas there are plenty of school buses serving state schools. I think that is a red herring.
OP, I can understand how if you live in Clapham you applied to Sutton and worried about the journey later - it is not as though it is easy to fill those six spaces.
I agree with others that it is doable, but think there may be a price in terms of doing things after school (leaving at 5, getting home at half six because you just missed the train, eating, starting work at 7, little break, finishing at 9, repeat.repeat.) I would do the journey for real at school times and time it accurately - and on the way back, just turn up at the station because the time he leaves school will vary a bit (depending on what he has lost or forgotten!). If you can wait to decide till schools open, and take a day off and do at least the morning one then, that would be even better.
I would look at hard at your third choice if that is closer, but if Sutton is head and shoulders a better fit for him, then go for it. I know people in your neck of the woods who turned down places that meant journeys like these when it came to it - so you don't see their kids at the station. So I don't think everyone considers it perfectly normal. It depends what your other options are.
Whichever school he goes to, now is the time to start letting him have some independence. If you start now, you can do it gradually and wherever he goes and wherever his friends live, he will be in a better position to cope.