As far as the UK is concerned, I don't think there's any problem with displaying a foreign (US) number plate. We get loads of continental registered vehicles driving on our roads, so I can't imagine you would have any difficulty.
You would, most likely, need to show a country of origin (i.e. the international code for your country) in the bottom left corner of the plate. As long as you are insured, you should be ready to go.
Don't take this as gospel, Steve - there's probably somebody else here better qualified than me to comment.
Have you considered hiring a bike instead - it'll save you a shed load of shipping costs?
