There aren't too many examples. Theoretically you can be charged but the cost to do so often means people will not bother to do so.
Certain places are protected under thier own respective laws for somewhat obvious reasons which can make trespass a chargeable offence under these acts (Nuclear sites, national railway systems and airports for example).
There are court injunctions which have been applied for in the past, as mentioned above. These are also costly and are often seen on places that haved attracted certain crowds. (Such as The Shard or certain power stations).
Most members will probably have some stories about police encounters and close calls on here. Often they're called to usher you away and assist in keeping the peace or they have been called with something else in mind such as a suspected burglary. Generally, once they realise what is going on, they'll tell you to get lost and go on about thier day.
Edit*
In short:
- Don't go anywhere stupid or that you're concerned about.
- Don't do anything stupid or obviously illegal or take anything that could obviously be used in such a way.
- Try not to get caught.
- If you get caught be calm, polite, honest and take the hit of mild embarrassment.