Thanks for all the comments so far, but Jackhandy's is really the only one which addresses my concerns. There is unlikely to be any legal problem with using an illegal vehicle until an incident occurs which invites the attention of the local police and my own (public liability) and the other party's insurance companies. If an electric bike does not comply with the local law the rider will be prosecuted for using a motor vehicle without licence, helmet, etc, etc, and will have his driving licence endorsed etc.
As for complying with the law in other EU countries, is it not the case that UK motorists driving in France have to comply with French law regarding carrying high visibility gear for use in the event of an incident? I suspect there are many other examples of the same thing, eg having to wear a "safety" helmet in Spain.
As for complying with the law in other EU countries, is it not the case that UK motorists driving in France have to comply with French law regarding carrying high visibility gear for use in the event of an incident? I suspect there are many other examples of the same thing, eg having to wear a "safety" helmet in Spain.