They usually invade common land, such as parks and recreation grounds own by local authorities Friday afternoon as they know nothing can be even started until Monday.
After that they have until emergency eviction notices from the courts.
However if they invade private land they can be made to leave by the police within 24 hours if the home owner (with police in attendance) tells them they have to go. Never, ever, regardless of what sob story they give allow even one night to leave. Once permission to stay, however short that time is, is permission.
Well that is what we were told and did when they invaded the local community centre I was a trustee at.