iCruise is correct, the SL-C3100 charged can be used with the UK 240V with a plug-adapter.
The Japanese manual says that it should only be used in Japan or Canada/US, but the charger is rated for 100-240v and runs safely for me here in Singapore with 240v.
I have a number of step-down transformers and tested the charger over extended periods (6 hours charges) both with and without the charger and didn't detect any difference in terms of the charger getting warm.
I accidentally used my iPaq charger at one point, and to be honest you are more likely to do damage to your Zaurus using a charge like this (which charges much more quickly than the slow Z charger).
If you are very concerned you can use with a step-down 240->110V adapter, but I no longer bother.
Regards,
Marcus