The easiest way to calculate is to find max Watts and divide by voltage. So in this case, its 500 divided by 250 = 2 amps. Also, you'll probably find the fuse on the mains plug is 5 amps, so that is the absolute max.(If there is a 13 amp fuse, then it needs changing to the lower rating as 13 is not protecting you. Your gear will have blown up before the fuse kicks in) Don't forget any backline amps or keyboards will all add to the load.
But in the UK, you'll be well within a 13 amp socket.
Most generators will have 16 amp or 32 amp Ceeform outlets (blue industrial connectors - NOT the yellow ones - they are only 110v), so you'll need the generator people to step you down to a 13 amp socket so you can plug in your system.
G