I'm not really comfortable with the notion that people drinking lager on trains are automatically miscreants, and particularly not if the implication is that they didn't have a first class ticket
It depends on the train. On a crowded Thameslink train, which was the example given, most "normal" people will stand in Standard. So people who don't care about rules and are willing to take a chance will sit in First if there's a seat. It doesn't matter if it is declassified First or not. These people just see an empty seat and sit down. There are virtually no ticket checks on Thameslink so it happens a lot.