If you feel that September would be the best time to take it, do it in September! You can still finish off your entire UCAS form as you don't include the UKCAT score in it.
If it's applying to your strengths that you're thinking about, you can make a list in advance of Uni's you'll apply to if you do well and which one's you'll apply to if you haven't, then finalise your decision once you've got your results. From what I understand, as long as you apply within the deadline it makes little difference whether you apply early or not.