power stations are considered criminal trespass i think?
Nnuclear power stations, in fact anything nuclear. They're all specifically listed here....
www.gov.uk
Most of these places will have armed security, and the police response will be armed.
Only nuclear power stations, or any with a high court injunction such as Fiddlers Ferry. The same applies to any site with a high court injunction. The whole of canary Wharf is covered by an injunction, also the Shard, and several construction sites acoss London, mostly Multiplex ones. You can be charged with being in contempt of court for breaching the injunction if caught on any of those, which can be quite serious. The guy who climbed the Shard got 6 months inside
With the injunctions, you'll probably only know when you get there and there's a copy of the injunction at various places. Technically it's still not a criminal offence to trespass at these sites, but what you're doing is breaking a court order. That carries a serious penalty and it will cost you 6 months in jail, or massive fines, in the tens of thousands.
A more likely tactic is that you'll be arrested for suspicion of burglary. The first time it happens it's unpleasant (after that you'll realise it's just a game and you can jerk them about as much as they jerk you about). That allows the police to remove you from the site and give you 24 hours in a cell. They'll take your fingerprints and DNA, interview you, eat some shitty food, and add the arrest to your police record, and in case you didn't have one, now you do.