I suppose the question is a bit like, "What's a good, cheap guitar," in that it all depends what you're looking for; but unlike a musical instrument, I'm not sure I've ever had a pair of trainers where my feet could tell the difference between a pair of really expensive ones and a moderately priced pair of last season's style on sale. I am American, so there are all sort of websites or stores where you can get really good trainers for a fraction of the price they cost in Europe, but that notwithstanding, picking a running shoe is all about what you're looking for. Are you a neutral runner, or do you overpronate? What kind of foot strike do you have (heel/midfoot/forefoot)? Do you run long distances or keep it short? Do you push the tempo or pace yourself? Do you weigh 75kg or 120kg? Do you have injury problems?
I find for my style of running (neutral, mid/forefoot strike, urban pavement and dirt, distances anywhere from 12km-30km), I can run comfortably in most trainers, but I like a low drop (most trainers have a roughly 10mm difference in height between the heel and toe—I like between 0mm-5mm) and medium/light weight. A lot of the most expensive trainers out there—Asics Kayano, Mizuno Wave Creation, even Nike Zoom Vomero (although I have a pair of those and love them), etc.—are very heavily padded and weigh quite a bit because of it. Either that, or they're a gimmick shoe like those Adidas trainers with the goofy plastic blades for soles or something. But moderate, all-purpose trainers can be quite affordable. I rarely spend more than $50-60 on a pair of running shoes.
Bottom line is, if you have special needs, take those into account. If you don't, you can probably get away with a lot of different shoes just fine. Check to make sure the size is right (Asics trainers tend not to fit me particularly well because their sizing runs long and narrow and I never seem to have enough toe room) and they feel good. I've run in Mizuno, Nike, INOV-8, Saucony, Asics, New Balance, and Salomon without any issues or much to choose between them at times. I got a friend a pair of Mizuno Wave Precision as a gift and he loves them. They cost $50 in the States, so they'll probably cost £75 in the UK. Nike Air Pegasus are great and less expensive than a lot of their fancier shoes. People like Brooks. My Asics DS Trainers were a nice middle-of-the-road shoe. Saucony Rides are nice, or the Kinvara if you're looking for less padding.
I'm probably not helping you much. I guess my advice is to do some basic research, and then try shoes on until you find something you like. There's definitely some trial and error in the process. If you have specific needs, I can probably make some recommendations with those in mind. Otherwise, good luck.