So you're basically asking if the Greeks knew about Britain and knew that it was inhabited? Of course, the Greeks were trading with Britain centuries before the Romans had ever heard of it. The earliest textual reference we have to the island directly is from Pytheas' Περὶ Ὠκεανοῦ, in which he describes (among other things) an expedition he made to Britain and later to Ireland to trade in ore--Pytheas calculated his longitude wrong, however so with his correct latitude he thought that Britain was much wider than it actually is. However, the Greeks almost certainly had known about Britain before the publication of the Περὶ Ὠκεανοῦ in the second half of the fourth century. Punic traders had been making their way to Britain for centuries before that, and it's very likely that sailors and traders from western Greek cities like Massalia (Pytheas' hometown) had known about Britain either from sailing out past the straits of Gibraltar or through their trade networks up the Rhone (by now well-established)