A good point, Khandro.
It's extremely difficult to get a successful prosecution for theft if someone takes something from a skip as the person taking it might reasonably assume that whoever put the item into the skip has no further interest in it and wouldn't mind if someone took it.
"A person’s appropriation of property belonging to another is not to be regarded as dishonest . . . if he appropriates the property in the belief that he would have the other’s consent if the other knew of the appropriation and the circumstances of it"
[Section 2, Theft Act 1968]
Note that it is completely irrelevant as to whether the owner of the property would, or would not, have consented to its removal from the skip. As long as the person taking it had a genuine belief that the owner wouldn't mind, he can't be found guilty of theft.