This is an excellent question. I don't have a particularly good answer for you (since it's certainly not my area of expertise) but - and maybe this will add a bit more to your question - a couple of years ago I went to Bath, England and visited the museum for the Roman Baths. I was surprised to find that a German craftsman made the journey from Germania all the way to Bath just to pay tribute to one of the gods being honored at the holy site.

Naturally this brings up a whole host of questions. In relation to your messenger, however, how common was it for Germans (or any one from the continent) to make it to the Islands and back? How arduous was the journey especially for such a non-governmental personnel?

Would a single messenger make the whole journey, or would it be relayed?