Aibu to use someone else's booker code? How do I get one?

[cultkid]

I want to get sweets they are cheaper there

Do they know you have their code? With permission no yanbu, without then yes you are.
Surely sweets aren't that important, it's not even loo roll or pasta

I've joined it legitimately instead

Have you checked if they're ACTUALLY cheaper there? I find that 90% of the time, normal supermarkets are cheaper than Booker