As long as you got an A or A* in GCSE then you should be fine, there was always maths support sessions for physics in my college, you would probably be able to find the same type of support too if you need it (the most you have to do is rearrange equations and substitute values or calculate gradients, how hard even is it but people still struggle). Id say just do a subject you find particularly interesting or fun (even something easy). Its not like all four of your subjects have to be relevant to the course you want to do, pick something youve done well in gcse maybe? Geography is more or less the same as gcse ive heard