Aurora R10, what's the appropriate plug fuse?

Fuses depend on your PSU output wattage rating,
Relevant Amps fuse rating should normally be clearly printed on power supply label on a "AC INPUT" line (both: Volts and Amps)
for 1000W Power supply 5Amps fuse could be ok while 2 Amps will not suffice.
Calculation is following: A=1000 Watts / 230Volts = 4.35 Amperes
for 500 Watts PSU 2.5 Amps might be ok (unless it's designed to handle higher peaks of a very short durations)
UK' (England) socket AC standard is 230 Volts (at 50 Hz) and most common fuses available in electric stores are 1A,2A,3A,5A,7A,10A,13A

Whatever the rating of the branch circuit is for the plug. The rating of what you plug into an outlet has nothing to do with the fusing for the circuit.

The power supply is fused internally and the power cable should be the UK AC version of the cable.

It depends on the current rating of the branch circuit, not the PSU output wattage rating.

You will typically have other outlets attached to the branch circuit, unless you ran a dedicated outlet for your PC.