If violence is normally the first resort, then the reasonable thing to do wouldn't be to personally intervene but note a description of the offender and what they're doing and when, and report it. I wouldn't tell them I was reporting them.
I'm not from the UK, just the Internet, and given it's not acceptable to call people gypsies anymore, I think traveller has become a de facto PC-acceptable term. It would be interesting to find out that accuracy has been exchanged for political correctness. Is Romany an acceptable word to use instead?