One factor to consider is money. With the college, will you get a loan? I was under the impression that you don't get loans for HNC/HND courses, and as you're a mature student I assume you will have to pay. How do the costs compare to the OU (which you can get a loan for)?
Do you plan on working? Working while you're studying with the OU is a lot easier than trying to work around a course that requires physical attendance. I've worked full-time throughout my whole OU experience, which has also meant I've been able to save up to buy a car and move out of my mom's house, which is something that hardly any of my friends who went to traditional full-time universities have managed to do. I have also got more 'proper' (i.e. non-retail/bar) work experience which has certainly worked in my favour and I was recently offered a position that was intended for a graduate, even though I don't graduate until the end of this year.
Secondly, 6 years is studying at a part-time rate of 60 points a year. I would recommend you start off with that, but you might find that you can speed it up a bit after that and finish your degree sooner. I did 90 points a year and finished in 4 years.
There are hundreds of universities in the UK. Of course you hear about Oxford and Cambridge a lot; they're generally regarded as the best. The OU is certainly far from the worst and generally OU students have a good reputation. In my experience employers are often impressed with OU students, especially those who have managed to study successfully while also working (or with other commitments, e.g. volunteering, caring for relatives, coping with disability or illness, etc).