Depends on the university. For some it's as soon as you get an offer. For others it's after Results Day when it's confirmed that you're going to that university.
what about deferred entry? would you wait to apply next year?
Do you think you would be at a disadvantage if you don't receive an offer till late so you are late applying for accommodation or do the unis sort it out once they ALL come back in to give everyone a fair chance??
I think the unis wait for them all to come in as opposed to a "first come first served" basis.