It's unlawful to take things from a skip or a bin because of section 1 of the Theft Act 1968
1 Basic definition of theft.
(1)A person is guilty of theft if he dishonestly appropriates property belonging to another with the intention of permanently depriving the other of it; and “thief” and “steal” shall be construed accordingly.
(2)It is immaterial whether the appropriation is made with a view to gain, or is made for the thief’s own benefit.
People sometimes get prosecuted for taking things from a bin or a skip.
https://www.bbc.co.uk/news/magazine-13037808
It's fairly obvious really. It maybe that you yourself want to sell the metal on in the skip. The fact it is in a bin or a skip at face value means that you intend it to be disposed of - it's not abandoned in law in the street. The disposer's intention is for it not be taken by others but to be destroyed with normal household rubbish or recycled. Like I said it's not legally abandoned.
Think of it this way, say you throw out your private love letters or printed emails into the bin. Your intention is for that to go to the tip and never be seen again. It's no answer for your neighbour to say - oh well I thought because the recycle bin was on the street for collection it was ok for me to have a rifle through it and get what I wanted.
[which btw is why you should cut up or shred any personal information including bills/invoices/ bank details before putting them in any bin that is left out for collection because people do go through bins]
but @IncognitoUsername yes it is a thing. scrap metal collectors are notorious for going round looking for metal anywhere. go to sleep and they'll whip off your glasses for the frames!