Probably somewhere earlier in this thread is my attempt to analyse this:
s1(1), Theft Act wrote: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.
The second part in bold is the wishy-washy argument for it not being stealing or theft. However, that probably isn't a legal argument. The "intention to permanently deprive" is clarified in s6
to include "treat[ing] the thing as his own to dispose of regardless of the other’s rights" - which probably includes copying, as that is restricted by the copyright owner's rights.
The legal argument is about the "property" bit. The information or articles (i.e. song, story, film) aren't property. Parts of the copyright are (due to silliness in the 80s), but the actual information cannot be property (see the link to Oxford v Moss above).
As for suing the MAFIAA for those silly "piracy is stealing" adverts; as far as I am aware, there is nothing illegal about lying on its own. It might be false advertising, but again, that doesn't seem to be illegal or unlawful of itself, but entirely up to the ASA to deal with.