From this: http://www.naturenet.net/law/common.htmlThe problem is that if someone is trespassing, they are unlikely to comply with a polite request to leave, and if they then do not, the landowner has little if any further recourse. Section 61 of the Criminal Justice and Public Order Act 1994 allows the senior police officer attending the scene of an incident involving a trespass or nuisance on land to order trespassers to leave the land and to remove their vehicles as soon as reasonably practicable. The power can only be used when there are two or more people there and "are present there with the common purpose of residing there for any period, [and] that reasonable steps have been taken by or on behalf of the occupier to ask them to leave" and either the trespassers have six or more vehicles between them, or they have caused damage to the land or to property on the land or used threatening, abusive or insulting words or behaviour - or both. So really it's not likely to cover anything other than a major invasion. This power is not often used, but for practical purposes this is the only instance where you might get the police to come and actually remove trespassers from a bit of land.
The only time you can use reasonable force to eject someone is if they have broken in to your property. If it is on land occupied by a shopping centre, the public have been invited in and are therefore given an implied licence to be there.