The insight, for which respect must be paid to screenwriter Al Ashton, is that these guys aren't poor white trash but professional men. Bex is an estate agent and when we first meet him he is selling a house to a couple by admitting to them frankly that it's rubbish. He shows them in and says "If this house don't sell itself I'm a monkey's uncle." Then he walks away down the path and, for a moment, scratches his armpits and gibbers like a chimp - an inspired bit of improv from Oldman.
This was Oldman before he got into his period of being an American Ham - sharp, keenly observational and immensely likeable even though the character he's playing is a complete scumbag. There's a lot of violence, and violence in a Clarke film isn't a rowdy punchup, it's Stanley knives in the face and iron bars in the groin. A gun gets used towards the end, which I personally found a bit unrealistic.
One of the most remarkable things about this movie is that at no point do you actually see a football. These guys aren't football fans, they're in it for the fighting. They were the energies that Margaret Thatcher unleashed and then affected to deplore. Guys like Bexy own much of Britain now.
When Oldman got tired of acting in bad American cop thrillers, he showed what he'd learned from Clarke by making Nil By Mouth. The boy done good. The Firm was Clarke's last film; a year later he was dead from cancer. Don't miss it.