They know that you are doing this and want to tie you in to the Indie (presumably by agreeing to a significant deposit / terms fees) in the knowledge that your child may well get a state school place.
I have quoted something elsewhere from a school website about this being contrary to some guidelines.... will try and find it.
edit: This what I found - haven't actually found the bumph from the ISC but it might be worth you phoning them. I remember a head of a school in Cambridge saying that they were bound by this.
edit: found this on beechwood's website:
As the Kent 11+ selection tests will now take place in September, some independent schools have moved their 11+ entrance tests forward to the Autumn Term, and will no doubt be offering places to successful applicants soon afterwards. Whenever testing takes place, however, the regulations of the Independent Schools Council clearly state that there should be no deadline for acceptance of places before the first week of March 2011.
At Beechwood, we have therefore decided to keep our Entrance Day in January, because it fits in better with our school year. We believe that this will still give parents plenty of time to come to a decision about their child's secondary education.