I would agree with the above post. Do what you love, but be realistic as to the job opportunities and what the demand is like for that particular job, as you are not just doing it for fun, but also to develop a career.
Look into adult learner loans, they are not available for general degrees, but if you go on the gov website you can download a list of all applicable courses. I am browsing through this list to see what I might like to study. This for me is a second chance to try a new career, so I can't complain!
6 years is a long time part time, and a lot can change in that amount of time. One thing I would say is that OU is not as cheap as it was back in the early 2000's. It's actually poor value for money as their prices are quite high these days, it seems better value to go to a physical uni where you can use resources, but I understand OU is useful for those who cannot work to certain timetables.