Just a word of warning the other way around...we have a confirmed private place, which we had to pay a terms fees for because we are out of catchment for our local state primary. We are not sure if this is the right school for our child, but we applied as an insurance.
We were offerred a place at our second choice state primary, which we accepted, which we had to if we wanted to find out more about the school. We then went to an open evening to make up our minds if this may be a good school for our son.
I mistakenly, in good faith, told the headmistress our choice, and that we were deciding by the end of the month. She reported us to the local authority, and we were sent a stern warning by the school that we had to confirm we were declining the private place!
We decided to turn down the state place, but it forced our hand to make an important decision rather quickly. We are still hoping for a late place to come up at our first choice state primary, which I hope will come up this summer. I reckon lots of parents will change choices in the summer. I know it is hard for the parents waiting for a place to come up, because I am one of them for my first choice school!
Sounds long-winded, but sad moral is not to be too open about all this.