They're very very wrong - the licensee and bar staff reserve the right to eject anyone and ban them from any bar for whatever reason (including the fact that they don't like someone). Bringing your own alcohol onto a licensed premises is illegal in the first place, let alone that they are U18.
I'm not speaking from a legal perspective here - but from my time working on bars. It's simple: you ask them to leave, you confiscate their alcohol (which you can do since it wasn't bought on the premises) and if they don't leave you call the police. The confiscation can get ugly, but it's pretty fun to take booze off people. Your licence only extends to drinking alcohol bought on the premises - if you didn't have the power to take this alcohol off people you would be in breach of licence all the time and your licensee would be in prison.