When I read this question (or is it a 'statement'?), I tried to avoid going into my usual 'nerd' mode and examining the legal aspects. But, because Wizard 66 has raised the point, I've taken a look at the legislation.
There seems to be a good case for arguing that the person taking the tin of Quality Street has, indeed, committed theft. Ethel correctly points out that nothing has been stolen from Morrison's but the person taking the tin has deprived Afcjan of a discount, which may mean that she is legally the victim of theft.
The Theft Act 1968 defines theft thus:
"A person is guilty of theft if he dishonestly appropriates property belonging to another with the intention of permanently depriving the other of it"
While, for most purposes, the tin didn't 'belong' to Afcjan, the law seems to allow for the type of circumstances described in her post by incorporating this definition:
"Property shall be regarded as belonging to any person having possession or control of it, or having in it any proprietary right or interest (not being an equitable interest arising only from an agreement to transfer or grant an interest)".
That would seem to suggest that theft has occurred,not from Morrison's, but from Afcjan.