Thinking about it, could it just partly be that they are islands and therefore very coastal. The Basques used to sail all over the place. So did the Irish, the Cornish, the Veneti, the vikings the saxons, the belgae must have done a bit to come over in sufficient force to beat up the other tribes. Maybe we're just the product of a hell of a lot of sailors down the ages doing what sailors do. Probably totally implausible, but worth thinking about all the same.