This has been copied from another forum I view.
Disclaimer: This information is provided with no guarantee of accuracy, and does not represent a legal opinion - if in doubt, consult a solicitor.
Civil Trespass: Civil trespass, also known as "simple" trespass, is not a criminal offence in the UK. You cannot be arrested for civil trespass, though police may attend if there's a possibility that another offence has been committed or will be committed. In England and Wales, you can be sued by the landowner in a civil court for trespass, in Scotland you can only be sued if actual damage was caused. Repeat trespass could be prevented if a landowner takes out an injunction, but this is unlikely in urbex situations.
Aggravated Trespass: This applies to trespass committed with the intent to intimidate or disrupt people taking part in a lawful activity. If you trespass on a golf course to move the flags, it's aggravated trespass. Generally, this does not concern urbexers. You cannot be prosecuted for aggravated trespass where there is no activity to disrupt, so it does not apply to derelict sites. It would only apply to live sites if you intended to cause disruption.
Criminal Trespass: One type of this offence applies to a specific list of sites, including defence sites, nuclear power stations and royal palaces. The full list can be found here. Another type applies to sites where bye-laws forbid trespass - these include MOD property, railway property, and perhaps other sites like power stations. There is no centralised list of bye-laws, unfortunately, but this site lists MOD bye-laws. A third type of criminal trespass applies to Sites of Special Scientific Interest.
From an urbex point-of-view, it is a good idea to stay away from any active military, nuclear, royal, utility or railway property - pretty much common sense.
Burglary: This applies to trespass with intent to steal, commit criminal damage, rape or inflict GBH. All the police need to do to justify arrest is to show that they reasonably expected that you had intent to commit one of these acts. Intent could cover such things as carrying tools, carrying a "swag" bag, or theoretically even taking pictures of valuable objects. It is unlikely that you would be charged, however, unless you did actually steal or cause criminal damage. From an urbex point-of-view, it is important to do nothing which would give the police any idea that you are there to do anything other than take pictures. Burglary can also be called Breaking and Entering or Housebreaking.
Theft: This should be blatantly obvious. It is a criminal offence to take anything which belongs to someone else. From an urbex point-of-view, it is a very bad idea to take anything whatsoever from any site. As an aside, it has been ruled that theft does not apply to information - photographing documents, plans, or indeed sites would not be classed as theft - though data protection, copyright or official secrets legislation may apply to some types of information.
Criminal Damage: This is where someone intentionally or recklessly causes damage to another person's property, or intends to do such damage. It covers such things as graffiti, arson, and vandalism. Intent includes posessing an item with the intention to use it to damage or destroy property - carrying a chisel for levering open windows, for example. From an urbex point-of-view, this means that you should never do any damage. Case law is mixed on details - for example temporarily unscrewing a panel to get in, then replacing it afterwards may not count as damage, but it's certainly not advisable.
Breach of the Peace: This is a very wide-ranging offence, covering any activity liable to cause distress or alarm to a member of the public. This could include refusing to leave a site when asked by security, or perhaps even showing off - swinging from a bridge, for example. In Scotland, the definition also includes "annoyance", making it even wider-ranging. From an urbex point-of-view, being polite and civil and leaving when asked will avoid most problems under this law.
Manufacture and Storage of Explosives: MSER 2005 covers any site used for the manufacture and/or storage of explosives, including fireworks. This makes it an offence to enter any site covered by MSER. Sites are covered by MSER until the Health & Safety Executive has determined that there are no explosives or explosives residues left on the site. From an urbex point-of-view, this covers any ordnance factories or explosives manufacturing sites that are still live, or which have not been completely decommissioned, and visiting them is not to be recommended.
This is not my own work but believed to be accurate.
Having said that though JP is out with no damage and LZ international worked. But it's usefull stuff to know for future references