| if you require 230v is it on 115v? Okay,|
If it were on 125 v and you fwd it 230 volts then the power supply would be toast and possible the motherboard with it.
It should be 12v U.S., but not certain of the voltage in the U.K.
It is the same the world over. 12v US is the same as 12v anywhere else. All PC power supplies have the same output voltages regardless of what the input voltage may be.
Testing a power supply with a multimeter will only give you limited information. That is that it probably works. The only positive information you are likely to get is that is doesn't work at all.
To test a PSU properly you need a PSU tester which introduces a load and ensures thatt the power supply can still produce the required voltage when it is working hard.