I am considering ordering a nook in the USA and taking it with me back to the UK. My concern is voltages. If I charge it via USB in the UK and it conforms to the US standard for voltage, will it be damaged or fry my Psu?
All the voltages inside a PC are the same regardless of where in the world you are. The only thing that changes is the input voltage fed to the PSU, which is then converted into the DC voltages required by the computer.
If you purchase a Nook in the US, you'll be able to charge it via USB from a computer in the UK or anywhere else in the world without any problems whatsoever.