My interest is in the organisation and supply of the Roman army in Dorset. This leads one straight away to their command of the sea and ability to muster hundreds of ships. Caesar assembled some 900 ships for his venture. Sir Barry Cunliffe and a few other writers emphasise the ancient and well developed sea traffic up and down the atlantic coast.
But we don't hear much about shipping in the ancient world because writers didn't go to sea and sailors didn't write.
I wonder if, after the crucifixion, some christians in Judea found themselves on the wanted list and decided to get out of the way.
Some roman author (I can't find the quotation) says that they had to flee to the ends of the empire. If you were in Jerusalem you could go east to Persia by land, with the risk of being checked by patrols on the road and perhaps an uncertain reception, or take a ship west to the Pillars of Hercules and then north. France was by then pretty roman so that wouldn't be safe. We don't know the date of the crucifixion but it was probably a decade or so before the second invasion of Britain.
The fugitives would have to sail on, stopping off perhaps in Ireland where the romans didn't have much traction.
I wonder if these fugitives didn't sow the early, discreet seeds of the Celtic Church?