If you've got a Lidl near you, then you really are set. Can't quite remember it off the top of my head, but iirc, they do something like 400g of lean mince beef for £2.50. Personally, what I do, is I go into Sainsburys and get loads of Basics Minced Beef. I know they are not lean as such, but it is an improvement over iceland's minced beef, and they are like £1.50 each and look and taste like minced beef. Don't get wrong, the frozen minced beef was still good; I practically lived off it for my first term of uni.
Other than that, I'd suggest having a look at your local butchers. They may or may not be more expensive, but if you look around, you might be a ; at the very least it would help you when looking at some other places to buy meat.
Definetly go into tescos about 45mins before closing time though, near the meat chillers. You can usually find stuff that has been discounted by a ridiculous amount. Co-Op food stores are also good for this apparently.