Thanks a lot for your response! I'm glad to hear that the materials so far are good. That's a big factor in my decision. I had been considering starting with Birkbeck, which has a Certificate of Higher Education in Physics and Maths. Physically attending classes would make me feel more confident, but it has its downsides, and doesn't leave me with the same option of second year entry afterwards.
I've heard a lot of positives about the Open University, and when I contacted Queen Mary, University of London about the programme they said that their OpenPlus students do very well. For me, it'll come down to confidence in my ability to home-study. It's helpful to hear from someone who's already doing it!