Theft is
(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.
So there has to be dishonest appropriation with the intention to permanently deprive. Picking the wrong bag up isn’t that - you haven’t got dishonest appropriation, let alone that with intent to deprive.
no dishonesty at the point of picking up the bag up, but once home and realised not his, and no effort made to return it i.e assuming ownership, is it not then dishonestly appropriated?
Theft is
(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.
So there has to be dishonest appropriation with the intention to permanently deprive. Picking the wrong bag up isn’t that - you haven’t got dishonest appropriation, let alone that with intent to deprive.