Good point, Chuck. (If it's only the switch which has that rating then the actual sign may well have a much lower power requirement).
If it's one of these, Vivkins3,
http://www.ebay.co.uk...s-Retro-/250901408155
then you've got two options.
You can either replace the supplied power adapter with a UK one which supplies 4.5V (which is what a bit of web searching suggests is actually needed) or buy a power adapter to plug the US device into. The former course of action requires that you take care over the polarity of the output whereas the second method simply requires one of these:
http://www.maplin.co....ltage-convertor-19413
Maplin Electronics is one of my favourite stores. That's not just because they sell 'big boys' toys' but also because their staff actually know what they're talking about. If you take your sign into one of their stores they'll be able to advise you.
Chris