I think your Alevel choices are good for the course. I had applied to Oxford but was rejected after interviews, so this advice may not be perfect, but here are some things I found helped and things I think I should have done.
Apply to UNIQ summer school when your in year 12 if your eligible to. I think applications usually open around December. It takes place over the Easter holiday and in July and August. It was so much fun and would definitely recommend applying to. Ive heard that other summer schools are good as well so look into them as well.
Start revising for the PAT test around the start of the summer holidays between yr 12/13. I was worried about starting earlier and running out of practice material, but there is plenty on the website.
Try send your application off a couple of weeks before the deadline. This allows time to finish any parts of the application not done by this time and you don’t have to panic about rushing it. Oxbridge unis have an earlier deadline, usually in mid October as opposed to January.
Try and visit on an open day if you can. This is not vital but is good to see whether you like the city and colleges, and perhaps pick a college preference to apply to (though not vital to pick a specific college).
Look at other unis to apply to. You get 5 choices on UCAS and whilst I still definitely recommend applying, the odds are still smaller than other unis and many good candidates have to be turned away as there is a limited number of places. I am now going to one of my other uni options and love the uni I am now going to just as much as I did Oxford. Open days are good for this but so are the websites.
Read around your course. Is there a specific part of physics you like? Maybe read books or listen to podcasts about it. It is a good talking point on the personal statement and is good fun to.
Believe in yourself. Applying to uni is an exciting and nervous time. Talking to students in the year above helped me.
This is just my own advice, my application in the end was unsuccessful, but thought this might help anyway. Good luck and enjoy Alevels