A yum yum is a type of glazed doughnut. It’s hard to see how a hybrid could exist unless it’s the shape. Yum yum s are usually square or rectangular, not circular.
It’s M&S trying to be cheeky, make little kids laugh. M&S is a prestige brand that has been losing customers in fashion. It’s increasingly being associated with old people, and they certainly do dominate.
The food however is of a very high standard, always been held in high regard. I stop off for food but I don’t bother with clothes. Their cheese scones are really good.