Mention is made regarding the machine owner being in the UK.
Surely a freeze plug-type insert block heater is available to match local electrical requirements. But..
If not, the heateer is nothing but a resistive heating element. And if operating at twice the US-nominal rated voltage then the unit will use only have the rated amps.
If a 120-vac unit was all I could find, I'd be inclinced to risk the money, change out the male plug to the local standard, test it in a water bath before installation, determine it works, install it, and forget about it until it someday fails.
I suspect it will work fine, even though we all know from experience that crap happens.
Another observation based on the foregoing posts: a block heater is the best thing since sliced bread for diesel engine winter starts. Saves the battery and the engine, over time. Add an on-board battery tender and you've got a primo cold weather machine (add also fuel cetane enhancer / degelling material for winter).
Please post back your continuing experiences so we may all learn.