I think Calais-Dover is the cheapest/shortest. But then you have to ride all the way through England to get to the meeting in Scotland.
A 2-way ferry with cabin and bike from IJmuiden (NL) to Newcastle is about € 300,-
'44 Liberator
For sentimental reasons... I have a picture somewhere of my father on the back of a Canadian bike messenger's HD during the liberation in March '45