This is from a UK perspective but it may affect other countries too.
There has been some discussion and worry recently in certain club chatter about taking historic vehicles abroad, the general opinion seems to be that if you drive your road legal registered classic on and off a ferry there is no problem, but if you trailer it it becomes "goods" and would need special paperwork and may need a "Carnet", these aren't a new idea but since Brexit this may bring the problem nearer and affect more of us.
My brother has recently brought back 4 bikes on an open trailer, ferry from Santander. No problems at all, one of the bikes does not yet have a number plate or V5, he is currently wading through that process.
