I'm just old enough to have done O levels, although they were talking about these new things called "GCSEs" coming along in the next couple of years when I went to school.
At O level I did maths (grade A), further maths (grade B), physics (grade A)then at A-level maths (B) further maths (C) and physics (A). I then went to a good university to study aeronautical engineering. I did get some remedial maths tuition in my upper sixth because I didn't find it easy.
IIRC, I then spent 2-3 nights a week, from about 11-3, throughout my first year, working on revision of my basic mathematical ability - I got through the first year, and the degree okay (although some of the second year maths still gives me nightmares), but if my maths had been any weaker than that, I'd not have survived.
Physics I never struggled that much with, because to be honest although the subject was mandatory, most of the genuinely important physics content was in the further maths syllabus anyway (Newtonian mechanics, 3d stressing, etc). So, although you need it, I'd not worry so much about it - it's maths that you live or die by.
I'll admit now to being on the books of three universities at present (one I teach at, one I research at, one pays my mortgage and I do both! all in aeronautical engineering) and teach from first year to doctoral level. The biggest headaches we all routinely get are lack of mathematical reasoning, poor written English, and poor research skills (particularly this strange view that all wisdom is on the internet). I've also heard several academics recently suggest that there might be mileage in dropping A level physics as a mandatory requirement for engineering, whilst possibly raising the grades for maths.
G