Well universities accept you based on teacher-predicted grades (due to the fact that you only get your actual grades in Aug which is too late). Teacher-predicted grades are heavily influenced by your mock grades. My college did two sets of mocks: one at the end of y12 and one at the start of y13. If you do well in the y12 ones you're practically set for good predicted grades. If you do badly, the ones at the start of y13 are a 2nd and final chance to redeem yourself. Otherwise, you might not get into the unis you want so I suggest you don't do what I did (i.e. cram the night before the mock) and revise like a proper fully functioning human being.