It may be that some independent schools are oblivious to allocation days. Just as independent parents live in their own little world so do independent schools.
They have every right to remove a place if it is not accepted before the deadline and lots of parents have to lose a deposit. Unfortunately this whole business is a bit of a game and we all have to play it. But, there is no harm in asking the independent school for an extension and it is perfectly fair and correct if they say no. There are other pupils who may have applied to independent schools only and currently do not have a place so accept a second choice, then to find that a GS pupil who accepted turned it down and they could have gone to a first choice had they known in enough time.