If I were a country at war with the UK, then I would offer free money to people when they register on their phones with their bank details. Once there is a sufficient number of people registered, a message can be sent to each of them offering more money to engage in terrorist attacks. Some of them will take up the offer and carry out drone strikes on key locations within the country. Simultaneous attacks on all banking data centres would take down electronic payment systems. Without the ability to buy food, society would descend into chaos quite quickly. They might wait until cash has completely gone out of circulation before they do this in order to achieve maximum effect.

Centralised digital banking is a security risk that will one day be exploited.