Are you sure? I can't find anything online that supports that. Many bars serve other drinks (vodka and orange, gin and tonic, etc) unmixed, so you can mix it to taste.
Buckfast is still sold, and that contains caffeine. Kahlúa too.
Ah, think you're right - this might just be Scotland where it's actually the law, but was certainly the common practice when I was working bars up north.
There were discussions here about legislating against alcoholic drinks with caffeine levels above a certain level but I don't think anything actually became law:
Buckfast is still sold, and that contains caffeine. Kahlúa too.