I'd be delighted to be proven wrong, but to the best of my knowledge the OU doesn't do an aero eng course. If they do, I'd love to tutor for it - it would be an interesting challenge.
Various of the "normal" universities do distance learning or part time aero-eng courses, and it's worth asking a few (they don't seem to publicise what they do much). Also Embry Riddle in the USA does distance aero-eng courses, they're not cheap but have a good reputation and don't seem to care where in the world you are.
G