Hey, first of all most Universities won't accept an access to higher education diploma for these subjects. I don't know enough about open uni pathways, but someone else might be able to give you some good advice. If you're really interested in maths or physics your best bet (in my opinion) is the pay the huge fee by high fee I mean take out a 24+ loan and enrol back into a local college taking maths, further maths and physics a-levels. Maths and further maths will give you a strong grounding to work with, and try to push for mechanics (m1, m2, and m3) modules for your maths exams as these will coincide with the physics a-level. However, you will need to know that this is a 2 year pathway and all the exams will now be at the end of your 2nd year which will only be a nightmare!
Sorry I can't be much more help for you, but all the best man.