Hey, train conductor in the UK here. There's honestly not as much red tape as you think, and it's not even a gray area. The way this is worded sounds like it's frowned upon but you can get away with it. The truth is, it's pretty much accepted as completely normal to drink on the trains, everybody does and nobody bat's an eyelid. It's completely standard practice to buy alcohol on a train or bring your own alcohol onto a train.

The only exceptions are if the train operating company stipulate they don't want alcohol consumption on their service, for example, Merseyrail do not allow alcohol. These are known as dry trains. The other exception is for what's known as special trains. These are often put on for football matches to transport fans to prevent hooliganism. Also often used for extremist activists to be transported, such as the EDL. These are usually dry trains. Apart from that, nobody gives a shit.