I did it in July which is relatively early. I felt back then I could have done with a bit more time and I regret not playing it safe and postponing the date, seeing as I didn't do at all well, although I'm not sure my mark would have changed drastically as a result of changing it anyway.
But with the UKCAT it helps to be completely at ease, with no doubts at the back of your mind so you can really concentrate on the task at hand, and I suppose that can be helped by going into the testing room comfortable with the amount of preparation you did.