Obviously the Romans carried on a fair amount of trade beyond their borders, but most of it was to the south and especially the east. But if I understand things correctly, very little of that trade was conducted by the Romans themselves. The silks and spices of the east and the gold and animals from sub-Saharan Africa were brought to the borders of the Empire by others and then purchased and traded further into the interior by Romans or, more likely, subjects of the Romans.

From Britannia, there are really only 3 potential trade sources beyond the border, as has already been noted: Ireland, Caledonia and Scandinavia. There does seem to have been a Roman trading post on the east coast of Ireland. What did the Irish have that the Romans might want (always an important question)? Ireland was believed to be rather rich in gold, so some must have come from there; wool, I suppose, and perhaps cattle, both of which the Irish had in fair amounts. But beyond that? Beyond a few luxury items like jewelry, the only other thing I can think of that Ireland might have to offer would be grain, but a) as far as I know the Irish were primarily herders rather than agriculturalists, and b) the Romans preferred common wheat (triticum aestivum which was grown in Britain and Gaul) and, for the lower classes, durum wheat (t. durum), while in Ireland any wheat that was grown was most likely spelt (t. spelta). I see no reason for any grain trade with Ireland.

The situation with Caledonia is similar, though without the gold. It seems to be generally accepted that there was trade with the Caledonians who lived south of the Firth-Clyde line, so there must have been something coming south. Mostly wool and meat I suppose.

What about Scandinavia? Certainly, Scandinavian trade became important in the late first millennium, but as Petraites suggests, that probably had a lot to do with the development of the long ship. Obviously the Germans from north of the Rhine mouths had ships capable of the journey across the North Sea, but they don’t seem to have been in the least bit interested in trade and probably had nothing the Romans wanted anyway. They and the Scandinavians were all pretty poor at the time. The German coasts were suffering from subsidence and flooding (which was one of the things driving their migrations) and the Scandinavians only began to really flourish after the sixth century (helped, if I remember correctly, but a general upswing in global temperatures that improved the agricultural traditions there).

So, to sum up, the Romans were perfectly willing to engage in trade with barbarians beyond their borders, even if they weren’t willing to travel very far. But for the Romans in Britain those barbarians had precious little that they might want.