As long as you have good GCSEs in Maths & English (Grade B or above for Maths) then the rest of your GCSEs don't really matter - although good science GCSEs help. In terms of A levels, definitely do Psychology if you can for obvious reasons! I did Chemistry & Biology in addition and although helpful, you don't need those subjects to get onto a degree. I massively struggled with the difficulty and workload of Chemistry & Biology, and would probably recommend Sociology instead. I also started off doing A level Maths and couldn't get my head round it at all so dropped out; but the statistics module on my degree are my favourite and my best results. In short: do "easy" a levels which universities will still accept.
Some universities require you to have at least one science A level, but this includes psychology. Have a look at a few university websites and what they look for in terms of A levels.