Kingmakers factions
The Gameplay Deep Dive and the developer blogs both put you on one side: Owain Glyndŵr’s Welsh rebellion against England. The enemies named are English barons and nobles, and Scotland appears as ground you conquer, not a side you pick. The developers have said they want new factions after Early Access, without saying whether you could play them. A choice between England, Scotland and Wales has not been announced.
You fight for Wales. No playable English or Scottish side has been announced. What would change this: a developer post or store text naming a playable faction other than Wales.

The sides, as the developers describe them
The Welsh rebellion
“In Kingmakers, you travel to 1403 to help Owain Glyndwr transform a Welsh rebellion into an empire.”
Redemption Road. Said in the tinyBuildGAMES video: Kingmakers - Gameplay Deep Dive
English lords
“English barons and nobles will throw massive armies at your settlements.”
Redemption Road. Said in the tinyBuildGAMES video: Kingmakers - Gameplay Deep Dive
Territory
“As you expand your conquest across England and Scotland, you’ll earn experience to permanently upgrade your character class”
Redemption Road. Said in the tinyBuildGAMES video: Kingmakers - Gameplay Deep Dive
The missions point the same way. The Deep Dive’s examples are besieging an enemy castle, hunting down an English spy and escorting Welsh noblemen, all in service of the rebellion.
“A mission might task you with sieging an enemy castle, chasing down an English spy, or escorting Welsh noblemen through your territory.”
Redemption Road. Said in the tinyBuildGAMES video: Kingmakers - Gameplay Deep Dive
Who you are in all this
Not a Welsh lord. You play someone from the future, sent back to change how the rebellion ends. The third developer blog puts it plainly:
“As an American mercenary, you are sent back to the 15th century to aid Glyndŵr’s rebellion and restore the original timeline.”
Redemption Road. From Steam announcement: Kingmakers Dev Blog #3, Picking a Fight with History
That timeline, and why the game’s version of history has Wales winning, is on Kingmakers story and setting. In co-op, every player is on that same side, each with an army of their own; see Kingmakers co-op and multiplayer.
New factions, wanted for later
“Post-Early Access, we want to add new factions such as the Scottish or French, have epic castle sieges, deeper troop officer systems, and maybe even a PvP mode.”
Redemption Road. From Steam announcement: Dev Blog #1, Meet the Troops You'll Command in Kingmakers
Two things that sentence does not say: whether those factions would be playable or would be new enemies, and when they would arrive beyond after Early Access. It is a wish list from a developer blog written before the delay. Where it sits among the other plans is on what Kingmakers Early Access includes.
Claims to be careful with
One on-screen detail is real: the mission-select screen shown in the Deep Dive is headed Welsh Campaign. It is pictured on the story page. What it does not tell you is whether other campaigns exist.