In the UK the household sockets are normally 13A so that would be the highest possible.

The rating for the computer itself needs more detail, but a quick check is to work from the rating of the power supply - say 500W
Power is E x I = 500W so 500/240 (UK) gives over two amps, so the nearest standard fuse would be 3A. It needs also to be the type of fuse specified, ptobably a slow blow that will allow for surge. If the same fuse feeds the monitor, you probably need 5A. (depending on monitor type)

Peripherals? What are they? Domestic printer, scanner etc probably 1A.

This assumes plug top fuses rather than internal fuses.

Stu