The power supply for the Companion 2 Series III is designed to work with 100-240v, 50/60Hz power inputs, so it will still work for you in the UK without any power converters being necessary. You would need an adapter for the plug but that should be all you require.
When you say the speakers aren't working at the moment, are they not turning on at all? Or do they turn on but produce no sound?
If they don't seem to be turning on at all, it would definitely be worth trying another power outlet with your speakers and removing them from any power strips in case this is stopping them from receiving enough power.