I suspect all independent schools want fees at Easter one term in advance, and then refund if you don't get the grades but NOT if you choose another school over them. The state schools however, will have waiting lists (if they are at all sought after) and there is nothing binding about any arrangement made when there are no fees!!! So the state schools essentially just wait for you to enrol, with the grades conditional on their offer, in August, until then it is up in the air really. There is a massive amount of fluidity in all the state schools round here in the first few weeks of Sixth Form, some internal students don't continue, some external students get bumped up the waiting lists, sometimes you can just apply at the beginning of term and get into some Sixth Form boarding schools (I certainly know people who have swopped schools at the last minute if they are from day to boarding, not sure whether other way round is so likely)
Dd is on the waiting list for a boarding school. I am dreading the letter when they ask me to make a decision and cough up the cash, I really want her to stay where she is (state day) Things seem rosier there than when the boarding option was mooted..
We have very academic state schools round here but unfortunately dd is not in the running for them, external students need lots of 7's and 8's, and she is just a bit bored at her existing school, and it is so far away too. But I know Sixth forms are so different from the rest of the school, so she will be pleasantly surprised if she just hangs on.