You've got plenty of time! I've just finished year 12 and Im doing my UKCAT in a few weeks and my BMAT in November, so you do them in the Summer before year 13 or during the start of year 13. The UKCAT is more so aptitude, such as situational judgement, decision making, using logic and reason, mathsy stuff and deducing conclusions. The BMAT is more science focused, it tests you on GCSE science knowledge, problem solving, a little bit of aptitude and you write a 30 minute essay to test your ability to communicate and argue effectively.