It seems that some people around here need to brush up on their law!
As long as you don't need to break in, you can lawfully enter anyone else's property at any time. (There are certain exceptions relating to, for example, railway property and military installations, but they're not relevant here). Trespass is only a civil matter, not a criminal one. (If someone leaves their front door open, you have every right to walk into their house and to look around)
You can lawfully appropriate property of another person "in the belief that he would have the other’s consent if the other knew of the appropriation and the circumstances of it". For example, if you see something thrown into a skip and you genuinely believe that the owner of the property wouldn't mind if you took it, you're not guilty of theft if you take it. (It's irrelevant as to whether the owner actually does mind. As long as you have a genuine belief that he wouldn't, you're in the clear).
http://www.legislatio...g/definition-of-theft
Chris