The Open University is pretty good for mathematics.
First year is a high school level - you do not have to take it if you are good at that maths. Second year courses are a bit more challenging, but real fun in on 3rd year. I did Complex Analysis, Fluid dynamics, Further Pure Mathematics - they are solid and interesting.
I thought about some physics like electromagnetism or quantum theory, but decided to stick with pure maths.
I think to also go get MSc with them - they introduced Galois Theory this year, although the choice of subjects is limited there.
I've also had some very disappointing tutors. One in particular was an arrogant arse whose own highest qualification was a BSc yet he was allowed to judge my Masters level assignments