Well, a couple of years ago there was that "Roman Fort" in Southern Ireland which the conservative archaeologists said was, rather, a private free enterprise emporium; frankly, it is hard to believe there would NOT have been trade across the Irish Sea which separates Britannia & Hibernia.

Scandinavia is a different matter, and your post alerts me to the fact that I have read of little contact between the British Isles and that part of Europe, except perhaps Bronze Age links between Norway and the Orkneys (which may never have even been settled by Celts). My guess is that the North Sea was hugely less navigable than the Irish Sea in the era before the invention of the longship... but this is very much an amateur guess.

PL