He might not have been able to secure control of Scotland for himself, but if Margaret of Norway then the Plantagenets may have been able to unite Britain, either under the then Edward II, or under his successor.
This has extremely interesting possibilities for later Anglo-French wars, as without the 'auld alliance' France is considerably weaker. The Plantagenets probably stand in a better place in the wars in France (which will probably still happen, given the French King's possession of lands they think are rightfully theirs) with a solid homefront and the additional manpower Scotland provides. This might not be enough for them to fully conquer France and establish a lasting Angevin Empire, but it would certainly help them keep a decent foothold on the continent (more than just Calais).
This also may inhibit the growth of Britain's colonial ambitions, as it would posses more European commitments.
While I agree that England and France will still come to blows over England's continental territories, note that the English won't have a claim on the French crown in this scenario (since that was through Edward II's OTL wife). Which is a two-edged sword: on the one hand, rebellious French lords won't be able to justify siding with the English under the figleaf of "supporting the rightful king", on the other hand, the later parts of the HYW might see less extravagant English wargoals.He might not have been able to secure control of Scotland for himself, but if Margaret of Norway then the Plantagenets may have been able to unite Britain, either under the then Edward II, or under his successor.
This has extremely interesting possibilities for later Anglo-French wars, as without the 'auld alliance' France is considerably weaker. The Plantagenets probably stand in a better place in the wars in France (which will probably still happen, given the French King's possession of lands they think are rightfully theirs) with a solid homefront and the additional manpower Scotland provides. This might not be enough for them to fully conquer France and establish a lasting Angevin Empire, but it would certainly help them keep a decent foothold on the continent (more than just Calais).
This also may inhibit the growth of Britain's colonial ambitions, as it would posses more European commitments.
AFAIK the Scottish* Lowlands were by this point seen as a part of Scotland. Borders based on ethnicity is a more modern concept.
(*= to avoid any confusion)
The lack of the Auld Alliance would help the English somewhat (the Scottish provided significant help both by launching invasions while the English were busy in France and by directly supplying troops to the French army), but on the other hand expect occasional Scottish rebellions to still distract the English.
To be clear, I am thinking the latter in regards to military conquest compared to a dynastic conquest. Basically as with Wales, English law and custom applied in a unitary kingdom.One issue that whether via the survival of the Maid of Norway or Edward I simply living longer and doing a better job of squashing the Scots during the Wars of Independence the geography of Scotland isn't going to change because Britain is under one crown.
Scotland is essentially split into three parts going from South to North, the Southern Uplands which is hilly rough terrain, the rich, arable central Belt and East Coast and then the Highlands. In OTL the Scottish Kings based themselves in the Central Belt and then faced endless problems controlling the both the Highlands and the Southern Uplands. The united British Crown is going to face exactly the same problems, the geographic and cultural distance between London and Inverness is even worse than between Inverness and Edinburgh so the Crown is going to have to concede a large amount of authority to local magnates in order to control the area. England had enough problems in OTL with rebellious magnates based in Northern England and North Wales, add more troublesome magnates in the Southern Uplands and the Highlands and the area strongly under the control of London (South of the Bristol/Hull line) has got no bigger but the problem areas are more numerous.
To be clear, I am thinking the latter in regards to military conquest compared to a dynastic conquest. Basically as with Wales, English law and custom applied in a unitary kingdom.
To push back a little, England was pretty centralised for the time and I would argue the unity of the island may aid the linking of the whole region. Rather than the fuzzy boarder region of OTL it would be one continuous area. And bearly anyone lives in the highlands so one would imagine a grand castle complex could do the trick of permanent pacification.