honestly honestly honestly I'd say focus on your maths and English. those GCSEs are pretty much essential for anything in the UK. you want to be a doctor? Keep at it at science, again securing good grades in those 5/6 subjects (English language especially) are incredibly important.
As to the other subjects, I'm not saying a grade c or d equivalent is good enough, but don't stress too much if you're not achieving the best of the best in them, especially if you wont be continuing them as A levels.
Prioritise in all honesty, that will greatly reduce the stress factor