sui
omg same, though I'm in year 11.
You want to pick the subjects that scales well like I'm taking spec, methods, GE, bio, physics and chem. Those set you up well. However, you also have to do well in these subjects so also pick subjects that you are good at.
In terms of UCAT, you can start anytime you like, the more you do, the better you are at it. I wouldn't start year 11 work if you haven't even gotten to year 10. You'll just burn yourself out, and i learnt this the hard way lol.