NFC, Near Field Communication. Built-in in most flagship phones.
In other words not in ALL phones out there (not by a looong shot). Plus the receiving end, like cash registers etc., need to be able to process it too.
So not enough enabled phones and not enough 'opposite' enabled receiving apparatus yet to make it a viable general payment system.
There are live trials with NFC-based payment systems in some countries on the European continent going on, or about to start this year.
Don't know about the UK.