i personally believe that you should take the subjects you most enjoy, as you will be spending a lot of your time studying something and you will be miserable if you pick subjects that you don't love! also seeing as you have an idea of what you would like to study at uni, your options seem like good choices!
that being said, taking a stem subject would leave your options more open, so if you happen to change your mind you aren't trapped with only a few university pathways - a levels are very different to gcses, and if you had asked me what i wanted to study in year 11 i would have given you a very different answer to now!
does your school let you take four a levels? if so, then i recommend starting the year off with the four you mentioned, and then you can see which ones you enjoy least and would want to drop - alternatively, you could attempt to do the full four a levels, or take one to as level and drop it after year 12 (if that is how it works in your school)
good luck in your gcses!