White/Black wasn't even the old UK colour code. Prior to 1977 it was Red - live, Black - neutral and Green earth/ground. Since 1977 it's been Brown - live, Blue - neutral and Green & Yellow - earth/ground.

White/Black/Green from what I can make out, is a US power cord. Neutral must be white or grey, Ground is green or green/yellow and live in theory is any other colour, though black is normally used.

Vox may simply have used the same cord for US and UK models.