Somebody else has already mentioned The Open University (https://open.ac.uk) and I happily recommend them, as one of their alumni.
Another established and well-regarded institution is the University of London International Programme (https://london.ac.uk). The courses are relatively low cost, but rely a lot on self-discipline.
I’m doing a Master’s with the University of London. I can recommend it though as with anything else you get out what you put in. I’ve found my course, Finance (Economic Policy[1]) interesting and reasonably rigorous.
Another established and well-regarded institution is the University of London International Programme (https://london.ac.uk). The courses are relatively low cost, but rely a lot on self-discipline.