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it being illegal to offer two prices?
Yes. Retailers aren't allow to charge extra depending on the payment method used. They can limit the form of payment methods they'll accept, but whatever forms of payment they do accept have to be all for the same price.
but ok to put the price up to £28 and sell it either for cash or via card transaction, rather than cash only for £25
No problem there. As long as they don't then turn round and say 'yeahbutnobut £25 cash is fine'.
it being illegal to offer two prices? but ok to put the price up to £28 and sell it either for cash or via card transaction, rather than cash only for £25