My view is, firstly yes they shouldnt have developed the site without obtaining planning permission first. That is against the law.
HOWEVER.....
Half of the site is completely legal & has planning permission. The other half (the illegal half) was a disused scrapyard, as I understand. From the aerial view Ive seen they seem to be respecting the boundary of the plot they own & not trespassing on neighbouring land.
As half of it is legal, this eviction can only hope to achieve at best a 50% reduction in the size of the site, at the cost of turning the travellers against the council & the local community, & causing tension in the community.
I think that yes while the council do have the right to enforce the clearance of the illegal half of the site, the common sense approach would be to grant planning permission retrospectively. They are allowed to do that, & by doing this they could have tried to build bridges with the travellers, as it is they have made enemies of them, not just the people they are displacing but also the remaining travellers who will be left living quite legally on the remaining half. I just think that the council have handled this very badly & after the clearance the village will not be a more pleasant place to live as a result.