If you receive a firm offer from a university, it will,be conditional/unconditional on results meeting a given criteria.
Sometimes you get better deal if you agree to firm early
Should you pass , and achieve the necessary points, any firm offer is a contract
Universities are independant bodies, with a governing body that adjudicates any disputes so that all uk units meet certain standards, but there isn't an education department controlling recruitment nationally
Just getting 5A's does not entitle you to a place at any University without them agreeing
In the same way, having a wallet full of cash does not force a shop to sell you stuff or a hotel to give you a bed
If a university takes more students than the agreed amount, they do not receive the additional funding.
That is why they make some unconditional offers and some conditional- to balance the books.
if they don't receive enough students, they may open more places or not hold the course.