My advice would definitely be smashing the UCAT in September. I’ve just applied this cycle, had an interview and waiting for a decision next month (hoping I get in 🤞)
As they are holistic I’d just try and be the best you can be in every area. Hopefully your GCSEs are pretty good already. Make sure you at least get A*A*A at the end of year 13 as this is the min offer requirement, it’d probably make you look a bit stronger if you’ve got all A*s already achieved by the time you apply. The UCAT will change next year (abstract reasoning section cut, so not a score out of 3600 anymore, but out of 2800). You’ll want to do as well as you can in this anyway to open options to all medical unis in the uk, but Cambridge will be looking for students with a higher than average UCAT score so really do focus on this.
In personal statement write about work experience obviously but Cambridge really values super curricular activities, things you do to research medicine outside of the curriculum because you enjoy it. Like an essay competition or read a book on a medical topic etc.
It looks like a lot but I assure you that you’ll be fine. Take each stage as it goes (exams -> UCAT -> statement -> apply -> interview hopefully). It’s a long process but it’ll be worthwhile if you get into Cambridge or not because of your determination and some of the things you’ll do and learn along the way
Hope this helps, and good luck!