Just pick the subjects you are likely to get the best results in. Yes maths and physics do help, but overall, employers are more interested to see those A's on your CV than a poor set of results that will not help in getting a job. Same in any job really!! If you have studied maths and physics at GCSE level and got half decent results, I don't see there to be an issue in sitting other exams at a higher level. Anyway most of the maths and physics you do is of GCSE standard!!