Oh yes, try and answer the actual question Genghis.
For any engineering career, you'll need maths and physics - for an apprenticeship towards a "technician engineer" role, the requirements will be rather less than for a degree towards chartered or incorporated engineer status. But for any of them, Ds in maths and physics are unlikely to get you anywhere - the courses are too mathematical, and the competition too tough.
Product design is a mickey mouse course at A level so far as most university engineering departments and engineering recruiters are concerned.
Computing is a serious subject, but only as a third A level. Even to get onto a decent product design degree course, they'll be looking for Cs in A2 for maths and physics, regardless of your product design A level grade.
In my opinion if you are getting bad maths and physics grades due to excessive time demands of the design A-level, either accept getting a much poorer grade in design, or just drop it - it's much much less useful to you than the "big 2". This is regardless of whether you want to be an engineering technician, a chartered engineer, a professional pilot, an air traffic controller or any point in between.
G