Meringue - yes of course they were "prepared to agree" at the time, because all nursery schools have similar ts and cs and if they didn't agree they'd have no place. Just because someone's agreed to something doesn't make it fair and in some cases the unfairness makes it unenforceable - as recognised by the legal rules against penalties and the unfair terms in consumer contracts regulations.
If the nursery was indeed unable to fill the place then I agree the parent who pulls out would and should have to pay. But we are talking about the situation where the nursery CAN fill the place but charges anyway - so basically they get a windfall as they get double payment for that single place.
Legally the argument would go:
- as a popular nursery school they should be able to fill the place
- given this, the term's fees is not a genuine pre-estimate of their loss and that term of the contract is invalid as a penalty and under the Unfair Terms in Consumer Contracts Regulations
- you are therefore not obliged to pay the fees under that clause
- you acknowledge that you must compensate them for any loss they in fact suffer as a result of your withdrawal
- however they have a duty to mitigate their loss, which means they need to take reasonable steps to attempt to fill the place
- so in order to be able to claim the fees from you they will need to demonstrate that despite reasonable steps they have been unable to fill the place
- you suggest that both sides discuss this issue again in September once it is known whether they have been able to fill the place or not.
NB the strength of this argument very much depends on how popular/oversubscribed the nursery is. If there is a real risk they won't be able to fill the place, then I wouldn't run this argument.
I presume you haven't actually paid the fees yet, but are being asked to do so now?
If you're going to run the argument above, and not pay the fees, you will need to be brave because it's possible they will sue you. However my guess is they won't dare as they won't want that clause tested in the courts.