Nothing, because they used analogue cable I believe (if it's a digital cable box it probably won't work either as Virgin Media hardware lock and encrypt their service, even FTA stuff going through their network I believe).
EDIT: In fact my nan had in her spare room an analogue Birmingham Cable box (one of these beauties)
And one day in 2004/5 it stopped working, she called Telewest as it was then out to look at it (as she had the Active Digital box in the living room, a horrible PACE thing she hated as it kept on overheating and dying) and they laughed and said it should have been replaced at some point in 2003!
EDIT 2: Just did some reading on this, Virgin Media began turning off Analogue in 2008 - alongside DSO, and by November 2013 there were no analogue services offered at all, withdrawing them totally from Milton Keynes and Westminster (where BT own the cables). Customers there can't get digital cable either from VM and BT don't offer the service!
Whoa. I had no idea General Instrument/Jerrold exported their boxes to the UK! We had a cable box like this in my parent's bedroom when I was little (through Comcast, as I'm in the US). I remember fiddling around with the remote and seeing the yellow text OSD a lot whenever I was in there (as compared to the other rooms in the house, which had ancient GI DCT1000 and DCT2000 boxes-- we were early adopters of digital cable). I saw one just like it in a thrift store a few years ago (obviously it can't work anymore as Comcast has upgraded their systems and everything needs some sort of box now).