I encounter the same sort of ignorance from customers on a regular basis, during such a till transaction I purposefully do not speak to the customer so as to avoid interrupting their conversation, which would be just as rude. Very often the customer will continue the whole transaction without the inconvenience of having to acknowledge they are being served, works for me, they can shove off and be rude and ignorant elsewhere, unless they refrain from carrying on their conversation they are treated as they treat me. The person on the sainsbury's checkout must have felt as ignored and insulted as I often do, but maybe she decided enough is enough. The store will no doubt reward the customer with vouchers and apologies but everyone knows the customer was in the wrong.