One of my friends goes to fcum games sometimes. He went to a big game (by their standards anyway) which they won. After the final whistle a few hundred ran onto the pitch to celebrate which happens a lot in the lower leagues and at small, basic grounds. Nobody there had any problems with it as the fans were not being aggressive. Though the police were, and one them decided go up to my mate (probably because he was small and an easy target) and give him a black eye!! The policeman didn't even try to arrest him as he knew he didn't do anything wrong. He just ran up to a 15 year old, bombed him and ran off.
I bet the copper would never even consider doing that to anyone outside a football ground.