I would just accept all sixth form offers, they don't share to my knowledge.
We have paid a deposit on two private sixth forms, one is a fallback depending on the exam results which we won't have till August- there is a risk she won't qualify for a conditional scholarship and if she doesn't get it, the other school is more worth the expense if we have to pay out all that money anyway. We aren't going to know which until results day, though I think she will get the required As in subjects she wants to do at A level for the scholarship.
One advantage of all this was it was her decision to apply and to sit for the entrance exams, though I suggested it and helped her register etc., and she was so amazed to get offers, it has really put the ball in her court more to study for her own sake, not because mum and dad are nagging. But we still do.
We have no intention of letting either school know she's not going to one of them- I'm sorry that will cause some reshuffling for others at a late stage, but dd was so worried she might mess up her exams that we agreed to do it that way. I'm sure the schools overbook and keep waiting lists anyhow. Why do they give out the results so late? Other countries get it done much earlier in the year.
We also have registered her to stay at her current school, that costs us nothing. I really don't think there will even be a ripple in that pond when she goes elsewhere, the teachers don't even know her name.