Permanent USB Power Off Mains
Transformer, rectifier, regulator - aka, a USB power supply.

There are, of course, the small switching supplies that plug in, as seen as phone, ipad, ipod (or any other USB device) chargers.

I know that there are power outlets (ac) with built-in USB outlets available for the USA system - depending on the speed and conservatism of the UK electric regulators, that sort of thing will either exist, or be coming at some point. Looks like it's a done deal: