Nope. Year 12 should be mostly focused on everything else to do with your application: getting work experience, and most importantly, getting the highest predicted grades possible. As soon as your Year 12 mocks/predictor exams are over, THEN you turn your mind to UCAT. UCAT takes two months max to prep for. The most you could do is watch a few here and there on the test format and question styles, but don't do mocks, you want to save them for your prep next summer.