I'm new to Reddit, so please excuse me if this question has been asked and answered before (I didn't see it in the FAQ). My understanding is that the bulk of Roman trade between Britain and the rest of the Roman Empire was conducted overland, with trade routes heading through France and only a short trip by ship across the English Channel. But pre-modern historians often make reference to the fact that it was much cheaper and quicker to ship goods by water than by land, which would seem to suggest that the more obvious trade route, at least for heavy goods, would be to take them by ship along the west coast of France and Spain into the Mediterranean. How much Atlantic shipping was there in the Roman period? If I'm correct that there wasn't much of it, why wasn't there more?
One reason may be that the journey through the straits and around Spain is a long and surprisingly difficult one. It's over 5000km, roughly ten times the overland distance. You're also on the Atlantic coast, which is considerably rougher than the Mediterranean (although I'm sure the more experienced seafarers could handle it). Comparing naval vs overland movement in this period, even a naval journey ten times longer might still be preferred. But if you have an overland trade route through friendly territory, that would weigh in its favour.
Moreover, I recently heard a comment about sailing through the Straits of Gibraltar. I'm pretty certain it was in the lecture linked here. The speaker comments that sailing West through the Straits of Gibraltar is easy, but going the other way the wind and currents are against you. So the crusader fleet which sailed from England was broken up and used for timber after it had served its purpose.
If we look ahead to Mediaeval times, the overland route to the Mediterranean remained key for English trade, and control of the south of France was economically very important for England.