Pain in the R's
I do suggest you buy some new socks, especially if you are ever travelling to Ireland. It has been the norm (for over a year now) for all passengers to be required to remove their shoes in Shannon and Dublin
Security could be stepped up for a number of reasons: recent review, new regulations, specific information from the authorities of a threat, general guidline from the authorities regarding an increased London/UK state of alertness, etc.
I hope they have extra staff or something to inform the queuing passengers at the security gate to be ready to take their shoes off (as well as all the regular stuff)...