Hi,
I have just finished taking 4 A Levels (Maths, Physics, Computer Science and Psychology). I also had a job in Year 12, and it was tough. I found pretty early on that I had to go from 20 hours to 12 hours a week, then WASNT getting any revision or homework done at that point still, so I dropped to 5 hours a week (bearing in mind I went to multiple sports/social clubs per week). In Year 13, I quit my job to have better focus on my revision. If you were going to get a job, it is tough, and I would highly recommend leaving in year 13 to focus entirely on your studies. Try the whole job thing and if it doesn’t work in your timing or you aren’t getting the work done or it’s too much pressure, then quit, at least you tried it and you will have a little bit of money.
Getting a job at this point in time can go on your CV, which will make you look better to Unis as well. When you start looking into unis, look at the grades too. I found that most unis took 3 grades, or ucas points. Then with that, I knew I wasn’t doing well in physics, so I focussed more revision on my other subjects (my course wanted 112 UCAS points, equivalent to a BBC or CCCD).