This would possibly tip me to buying the game to be honest, it would allow me to play a british isles tsl for a start (and i have seen tsl maps appearing already)From my perspective, the likely, simplest "all-Briton" full path would be Celts (wide, I know) for Antiquity, England for Exploration, and Great Britain for Modern.
The Antiquity slot can vary if they feel like innovating with other similar cultures (i.e. Boudica and the Iceni), but the existence of an Exploration Age Spain practically necessitates a contemporary England*. Possibly along with Francis Drake, taking advantage of the new "actual rulers optional" paradigm. Oliver Cromwell would be an interesting leader, too, but I wouldn't go as far as predicting his inclusion.
I'm not sure Wales would be particularly worthy, but Exploration Age Scotland would likely capture the period of most strife with its southern neighbor, assuming its inclusion, of course.
*The certain existence of the Normans creates a bit of a pickle there, though...
I can see other posters discussing historical accuracy, but to be honest all that will matter to the developers is market potential (that is why they left Britain out for dlc, to sell dlc).
I have no idea if there is a demand for more british isle civs at all. To be honest the price of a civ and leader seems even more extortionate when you consider the civ only lasts 1/3 of a game.