We usually recommend to our UKCAT students that they sit their test earlier rather than later, especially if going into year 13. You don't want to have to juggle UKCAT on top of starting A2, submitting your personal statement, maybe doing BMAT etc. It is also good to try and book your test well in advance as spaces can fill up quickly and the test can always be postponed up until quite soon before the date.
Best of luck and let us know if you would like any more pointers/advice!
Alex, 4th year UCL medic
6med