Hey,
I have already done my UCAT and I got 2940 and a band 2.
I did a UCAT test (on the website in real conditions) in October with zero prep to see what I would get then I started looking at UCAT question bank (also on website) to familiarize myself with the different types of questions. I was only doing 30 mins per section a week and not doing it timed or anything like that. I then did another test in December and after that I started to time myself, try different techniques and increase the time to 1 hour. In March, I did another UCAT test then increased the time by 15 mins and did a last UCAT test in July. I didn't do much UCAT during my mocks and during the beginning of the summer holiday as I had work experience. The UCAT tests allowed me to see if I made any progress and where to improve.
I used the UCAT question bank and 'Get into medical school: Master the UCAT' book during the school year. Then after mocks I used Medify. I found the book really useful because it explained the different question types and different methods.
I think it would be fine to start now as long as you can manage your time well. If you also think you would get really nervous in the holidays then I would recommend starting now so you have more preparation time. The UCAT itself isn't necessarily difficult in terms of questions but it is the timing that is hard so if you feel like that is an area which you will struggle with, it may be useful to start now. Also, you might want to practice typing in numbers and using the calculator in the UCAT test because it is really annoying.
However that being said, I know of someone who started after mocks and got around 2900. So it depends on each person. Also, I did not get burned out.
Hope this helps and good luck!