This is all very theoretical at the moment, but worth asking if anyone else has done this (or indeed if it's common practice!).
Given that the dates you have to accept private school places at are early in March, and the dates that state school allocations come out are in May, is it reasonable to accept a place at a private school, then if you're allocated the state school you really want, to withdraw from the private school (and lose the deposit)?
Or if you're applying to private schools, are you not allowed to apply to state schools full stop?
We'd like DD to go to a particular primary school, and if she doesn't get in we would potentially send her to private (various reasons), but would have to accept the private place without knowing if she'd got into the primary. Is this a common situation?