I'd lay you odds none of the evhicles are taxed/tested/insured - so why are they just allowed to drive around with impunity when you or I would be ANPR'd and clamped?
They seem to be able to get away with breaking the law. When they invaded Skegness over 20 years ago the police seemed powerless to intervene. The town just closed down. We were afraid to go into town, it was dreadful.
The argument against impounding vehicles when they are illegally 'camped' is that it would give them an excuse not to leave ...
... so follow them until they are in the back end of nowhere and then impound all the nice Mercs and Beemers - but that's too simple (and too much like hard work) ...
A theory I have had for a few years is that if the Travellers' vehicles were impounded it may make them homeless and they would then become the responsiblity of the Local Authority, housing, schools, etc. Much easier to let them move on and become someone elses problem...