Prompt/Idea/Challenge for the People!
In 1286 King Alexander III of Scotland died, leaving his 3 year old granddaughter Margaret as heir, with an agreement that she would be married to Edward, son of Edward I, being made in 1290, however she died not long after arriving in Scotland, creating a succession crisis that among other things would result in the Scottish Wars of Independence and creation of the Auld Alliance with France.
What if she didn't die? What if she lived to marry Edward and have children with him, binding the Scottish and English crowns together in alliance through blood? While the agreement included a clause that Scotland would stay completely independent, in practice it would be very difficult to (even if she alone would be crowned) - the King Consort of Scotland, would also become the King of England, while the Queen of Scotland would become the Queen Consort of England, functionally bringing the two into a Union of Crowns, so both would be dependent on the other.
What effect would all this have? How would Edward and Margaret rule together? What would happen with Gaveston? No marriage with Isabella so would the tensions flare up sooner with France? No Auld Alliance along with a safe northern border with Scotland means England can focus it's forces on France in the event of war (and even possibly have help from the Scottish, could even see a William Wallace fighting with the English against the French). No Scottish Wars of Independance, so far far less bad blood between the English and Scottish - quite the opposite if their monarchs are married and then in the future are ruled by the same person with the blood of both in a (relatively) peaceful process. If they stay together through several monarchs could have a much earlier Kingdom of Britannia/Great Britain.
If it eventually results in a 100 years war full victory for England you could have Scotland, England, and France in a Union of Crowns, or England retaining holding on the continent if they don't manage a full victory but still win. Earlier conquest of Ireland and perhaps even a splitting of it - north Ireland to the Scottish while south Ireland to the English?
Then with it all being much earlier they could be a lot more assimilation between all 4 which could result in a more solid British cultural identity - less English dominance and Irish & Scottish resistance and more equal willing mix (especially between Scots and English) perhaps even resulting in the British culture, and a British language, becoming predominant over the English/Scottish/Irish/Welsh ones, where I imagine (though I am in no way a linguist) the British Language would be mainly English but with more Celtic/Gaelic influence than French (and German) then the otherway around as was OTL.
What do you guys think? Thoughts?