Struggling to get a reference as a mature student

I'm a mature student and will be applying to university this year. My first choice is Imperial to study Maths. Finding a referee for my UCAS application has been quite challenging. My old college isn't an option, and so an academic reference doesn't seem possible. I used to work part-time at a supermarket, so I guess I could ask my old manager for one. The issue is, I'm aware that universities prefer academic references with predicted grades, so I'm worried that this could hinder my application. I'm applying to a fairly competitive course at a fairly competitive university, so I wouldn't want to lower my chances any further. Any advice?

Why is your old college not an option? As you say, assuming you were there fairly recently (i.e. you're a young mature student) they'd be ideal.
Is the reason you don't want to contact them worth the impact on your application?

I left the college quite a while ago (7 years ago) and I left at the end of year 12 so I doubt that they even wrote me a reference.

Agreed. They won't have written a reference for you if you didn't even start year 13, and seven years is really too long ago to expect them to be able to write one now.
Is there any chance that you'll be using a tutor to help you study for your A levels? If not, could you source a tutor simply to act as a referee and provide predicted grades?
By the way, your comment that you'll be "applying to a fairly competitive course at a fairly competitive university" undersells it somewhat. In recent years, Imperial have made offers to only 3 in 10 BSc Mathematics applicants, and of those who ultimately start the course, the most common grade profile is A*A*A* (source: the "Historical entry grades data" section on this UCAS page). That's sounds very competitive to me!
Also, I note that you've posted this in the Cambridge forum, which doesn't really seem like a good forum for it. Would you prefer that I moved it to the Imperial College London forum or to the Mature Students forum?

I will be using a tutor, but does the tutor need to be a PGCE trained teacher in order to be able to provide predicted grades? Most of the reasonably priced tutors are uni students, so I was a bit hesistant.