What do I need to do as a potential mature student in the UK?
I went to the UK to start my undergraduate when I was 27.  I was also not able to access the normal 'go-to-class' higher education courses.  My path wasn't computer science, but I think the general rules and requirements are roughly the same for each course.

In a mature student, UK universities are typically looking for enthusiasm, evidence that they are interested in the subject they are applying for, and that they are capable of doing university work.  Ideally what they want to see is that you have done some systematic course of study, typically in the last three years.  You have this (A-Levels and the diploma).  They also value experience - which you are getting by being in work in the area you want to pursue a degree in.  

Your A-Level results aren't bad at all, but as you note fall short of what they usually want.   With your work experience and additional formal certification, it is possible that the universities will look more favourably on your application.  Remember that even if you were to go into university straight from A-Level, grade requirements aren't hard and fast rules.  Which leads me to the next bit that is important - the personal statement.  This is the point at which you can say that you are more than capable of doing the course.  Don't dwell on your A-Level results, they can already see them.  Instead you sell yourself.  Really- this can make an application very attractive!  It's not all about grades - especially when you are a mature student.

If you did want to pursue another formal course, have you looked at the Open University options?  I did one of their courses (one 10 credit course) as I had been nearly 10 years out of education, and quite frankly my A-Level results were not...great.  It needed a lot of commitment to study and work full time, but overall it was a good experience, and I felt the course was delivered well.  If you can find something there that is relevant, I can highly recommend them.