Give the uni you want to attend to a phone call before you do anything. Dont e-mail them as that will take to long and you most likely wont get the kind of response your looking for.
Explain your situation and say that you dont quite have the A level grades. The uni should be able to give you an over view of what you can and cant do. You might even be lucky enough that the course you want to do has spaces, meaning that you can instant transfer this year and just be a few weeks behind. Eitherway dont leave it to late, uni is expensive and its best to get things organised as quick as possible and figure out what your options are. It might be an idea to speak to your current uni, they might be able put you on the course you want to be on now, or help you out with obtaining the A levels you want.
Good luck.