Any person can enter a place if the landowners permit it. However, this does not necessarily make a permanent right of access, and unless they have dedicated a bit of land to be permanently open it is within the power of the landowner or their agent/representative to ask any person to leave, assuming that person does not have some other lawful reason to be there.
The landowner or their agent/representative does not have to give a reason. If the person does not go immediately, by the shortest practical route, then they are trespassing. Despite the well known sign ‘trespassers will be prosecuted’, trespass is not a criminal offence and trespassers cannot usually be prosecuted. They can, however, be sued in a civil court. To be successful in court the landowner must prove a significant financial loss as a result of the trespass and thus it's extremely unlikely that this would ever happen.
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There are things that can turn trespass into a criminal activity, namely causing criminal damage, taking items away from a site, and threatening behaviour towards staff/security (this can be verbal as well as physical).
Criminal Trespass is a new law introduced in the Serious Organised Crime and Police Act 2005 and makes any trespass on Crown land a criminal offence. The law was introduced primarily to stop protestors disrupting military actions or invading places like Buckingham Palace & The Houses of Parliament. However the law applies to any piece of land owned by the Crown (including MoD sites). The penalty for criminal trespass is up to 6 months in prison with up to a £5000 fine.
http://www.derelictplaces.co.uk/index.php?do=trespassAs the lawyer says in the article:
No one has an automatic right of entry to someone else's property without consent. That applies to police officers and members of the public, and whether it is unsecured is irrelevant. Police need a warrant from a magistrates court if they wish to do this, or they may enter a property if they believe an offence is being committed. To my mind, if they have entered properties like this, they are trespassers. Trespassing is not a criminal offence but it is considered a civil wrong and could be pursued through the courts as such."
He added: "What if one of the homeowners goes on to claim something has gone missing while the officers were in the property? There is a risk of an accusation of burglary. And everyone is entitled to use reasonable force to defend their property from intruders. Conceivably an officer could have been attacked."
So it's possible there might be some finding against the police in a court, but it's not clear. 'Reasonable force' probably wouldn't include shooting someone dressed as a policeman with no weapons when they're not threatening you physically in any way at all, I'd think, though that would be up to a jury. Perhaps a jury might accept a rugby tackle and a few punches and kicks as "reasonable force to defend their property".