It sucks severely that the last few years British exams have been so badly managed to induce undue stress on those taking their exams. I was lucky enough to do my A-levels before this started, yet still suffer from exam anxiety because until my last set of university exams (and my first year of college), I'd never taken a exam seriously and thus performed at far less than my best.
My advice is two-fold;
1. Always do your best, that way you can never blame yourself and even if you don't get what you wanted it is far better to have done your best and failed than not to have done your best and failed (I use the term failed loosely).
2. Never matter what you got, my A levels sucked and it was completely my own fault but with drive and ambition you can always achieve what you want even if it takes you a little longer.
Just remember do your best and you'll have no regrets - that's what I've learned, wish I'd do it though

.
If you believe them miss-marked, take a year out, earn some money (you'll need it!), and don't give up on your first choice university because some underpaid, overworked teacher has been given another time-consuming, important duty and has for thanks been landed as a scapegoat for the governments ineptness.