Practise is really the way forward with the UKCAT. Having said that, the normal advise given to students sitting the UKCAT would be to do 2-3 weeks of practise, 4 weeks at most. So you still have a LOT of time before you have to think about this stuff. I would say that the best time to do the UKCAT would probably be around end of July/ early August, simply because it gives you enough time to prepare for it after school finishes and doesn't take up too much of your summer. I would also advise putting off writing your finalised personal statement until you have done the UKCAT, simply because, once you have your UKCAT score you would have a much better idea of what unis you can/ want to apply to because you'll be able to assess your own academics (GCSEs, UKCAT, A level predictions, work experience etc) against each uni criteria and then really tailor your personal statement to those unis.
Personally I'd recommend Medify - they have a question bank online and you can pay for access to these questions for a limited time (I think for two weeks it was £30). I'd also recommend the Kaplan ONLINE course (not the 2 day classroom course) - This is more on the pricey side with the online course costing £170 and the classroom course costing over £300 (I think). But these resources really helped me when I took the UKCAT.
The reason I recommend the online course instead of the classroom course, is not because the latter is bad at preparing you, I just don't think it's particularly worth the extra money because for both you get access to the online resources that they have - and that's what is gonna help you 'master the UKCAT'.
There are also a bunch of current medical students from cambridge, ucl, imperial etc. who offer UKCAT courses online, I think they go by the name 6med (but not too sure), I never used them personally, but they did look like they had good resources and are definitely cheaper than the Kaplan course.
I can almost 100% guarantee that I would not have done nearly as well as I did when I took the UKCAT if I hadn't done these online courses - but again, this only my experience.
Equally, many people perform well without forking out so much money, but I think having that practise with answering questions on a computer as opposed to answering questions from a book really does make a difference. I had friends that used both Medify and the Kaplan course, and friends who only used Medify and they got very similar scores. Because ultimately, the UKCAT really isn't that hard, especially for prospective medical and dental students, but the reason it can be so challenging is because it's time restricted and practising the test online can really help with that.