Preston Market has excellent prices for fresh meat and fish. The vegetables are also well priced. The market has clothing stalls too, but really it's the food I visit for. I grew up in the area and even though I'm on the other side of town, there is not anything to compare it to.
Just go and look! Like most markets, there will be markdowns of fresh fruit and veg towards the end of the day. Not sure about the meat or fish.