Can the power supply be replaced on these powered audio speakers?
It looks to me like the monitors are 100W each, and the subwoofer is 160W i.e. 360W total. 300W step-downs cost around £40 each on amazon, so if you bought 2 that would be £80 and cover everything you need. 

Those manuals list different versions for different countries/voltages, so it's likely that the only difference is the input transformer that they use. If you could get a transformer from the same speaker for the uk region (wj271700 monitor and xw546a00 subwoofer) then it seems fairly easy to install it. 

I googled and didnt find anything, maybe you could buy one from the manufacturer or buy a broken speaker and salvage it. Paying someone to install it + buying it will certainly cost >>£80.

Finding a compatible transformer will be difficult. Not only do these things have multiple different voltage outputs, this is an audio application - you'll always be wondering if the sound coming out of your £500-each speakers has been tainted by your mortal hands.