Payment-processing failures at several high-profile retail brands in the UK over the past week disrupted on-site customer service and stirred speculation about the cause of the outages. The problems at fast food restaurant McDonalds, supermarkets Tesco and Sainsbury’s, and bakery chain Greggs, highlight retailers’ increasing reliance on third-party payment systems and the technical issues hampering a global shift from cash to digital payments. All the affected retailers had problems with order-processing or accepting contactless payments in the last several days, causing locations to close or …