Uni

Of course not, you can take a gap year and apply the following year or you can drop down to clearing on results day and see what uni's still have places

Do Oxbridge, LSE imperial etc. do this too? and If this was to happen would it be better to take a gap year or do the clearance. I'm currently doing AS and I'm really upset about how I messed up this year

No, the top top uni's usually don't have spaces left on clearance. Let's say you get 3 A*'s, the best option for you would be to take a gap year and apply the following year. If you really think you can get 3A*s then go to your teachers and negotiate with them and tell them how you believe you can achieve better grades and you will try harder next year to get those grades.

If I take the gap year, do I apply as normal with my A2 grades?

Yes it is the same procedure, and instead of predicted grades you give your actual grades.

How does clearing work can someone explain pls

Its when on results day you call other unis to tell them that you got the grade requirements. If theres free spaces for your course then they will take you