As far as voltage is concerned, power supplies come in three types:
#1. A unit designed for a single voltage -- 120V in the U.S., 240V in the U.K. In this case you would need to replace the unit with the appropriate type, or buy a separate transformer to run it on the higher British voltage.
#2. A unit which is switchable to either 120 or 240V. Just find the selector switch and change it, then use a suitable power cord or adapter.
#3. A unit which will automatically adjust to run on 120 or 240V (and any voltage between). Many of the latest units are of this type, so all you need to worry about is the power cord/plug.
Just check that the power supply for the computer you are considering will run on both 120V and 240V 50/60Hz and it will work fine in both the U.S. and the U.K.