Do you know why you are getting U's? Is it because you haven't done enough revision or because you've missed lessons or because you don't understand the content or you struggle with how the questions are asked and structuring the answers correctly for example? I think you need to look at why you got U's and work from there.
Go through past papers, write a lot of practice essays and essay plans, It'll be tough and you'll need to put the work in - but from a U the only way is up!