At most of the car shows Ive been to there are number plate sellers who will do this providing you sign a declaration that they are for show use only and not to be used on the roads.
I guess if you then use them on the roads its your decision and your risk of presecution
I've bought 'show plates' for my '63 Dodge (California black-plate style) and didn't have to sign any kind of disclaimer about using them on the road. But that was a few years ago - so maybe the law's changed?
Never been pulled by the fuzz (oo-er) when using them tho