hi there caustica,
you're lucky, those schools are all excellent. as for atmosphere, they will all be packed with sloaney types but that is o.k. just be yourself. you'll only be teased about your background if you seem prickly about it. o.k., i think cheltenham is the pick of the crop re. snob value. headington i think might be slightly more normal in terms of its intake. but i wouldn't worry too much, just try to get into one of them. and religious studies A level without a gcse, that is fine. no problem at all.
a bit like school so you get help
sometimes you get a nice uniform
most things are done for you
easy to get around
close friendship groups
you may hate the people going
you get treated like a baby
you cant get away with much
you have to stop in school all day even during free's
its like normal school just different uniform
However it does depend on the one you decide to go attend.
Pro: No annoying little kids that seem to get more annoying with every year group (trust me, I've seen it)