Check the voltage range printed on the unit.
Some modern power pack units only need the pass-though type of physical only adaptor, but if the voltage range doesn't go up to the 240-250 volt range, then you would need a step-down transformer type adapter.
One the one hand, these 'universal' power packs are very common these days, but on the other hand a lot of goods out of Asia are mislabeled, so it's probably better to just replace the power pack with one from a U.K. supplier.
All the PRS-x50 readers use a VERY common USB style charger, and I see U.K. compatible ones on Amazon and elsewhere for only a few Pounds.
The only thing to be careful about if you do go to buy one is that some older 'Apple' type micro-usb chargers don't work.
About 99% of current cell phone micro-usb chargers do work just fine though, so if you have one of those available you are in great shape and can just use that.
