Thanks Devoid.
Well this is the idea that I had in mind. POD would be a fair bit before the actual OTL war.
This we have some original colonies missing/divided. As well as Hispaniola being successfully taken by the British.
But I had no idea on how to divide the lands west of the Mississippi, outside of Texas breaking free from Mexico and joining the Empire.
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My humble personal opinion as somebody who lives in a country that was mostly colonised by Britain in the 19th century is that any surviving British North America which expands past the Mississippi will probably have a lot of subdivisions there defined by boring latitudinal and longitudinal straight lines, just like the IRL US states and territories there. Easier to send out surveyors with equipment for and have them stick to while the area is still in the process of being explored, tamed and settled.
If they went to the trouble of doing this in uninviting semidesert on a continent with an inland region much less well-known and traversable than North America's (which has the blessing of navigable waterways and is not the dryest continent on the planet except for Antarctica), I am sure they will be able to do the same in the American interior. Also, it'd be much like actual US surveyors on expeditions did for the OTL US state borders beyond the Appalachians. In both the Australian and American cases the lines came out slightly wonky but were generally straight as intended. Both countries even had court cases over their slightly-wonky internal borders (colonial South Australia vs Victoria, and numerous US states), so the parallels are strong across the Anglosphere.
Plus a lot of the 13 colonies had charters theoretically extending their western frontiers all the way to the Pacific Ocean between certain degrees of latitude already, ergo straight lines were the norm. IIRC Virginia claimed, west of the Mississippi, a slice of territory from the 40th Parallel North (current northern Kansas border) down to its southern border extended all the way to California and the Pacific. Obviously we can never really know what the British would truly have done in such a scenario, but would it really be that unrealistic or uncharacteristic of them to detach that and make a long Missouri-Kansas all the way to the Rockies? I imagine these borders will be drawn up in London as well, or with London's approval (as the Galloway Plan of Union, for example, would have required for most peacetime acts of the American government) , like the Australian colonies' borders (defined by latitude, longitude and, to the lesser extent which they exist in Australia, big rivers) were. The only reason I can imagine London not wanting to define distant colonies' boundaries in this manner is if they're still trying to accommodate the slappabitch and kickahoe tribes like they did in 1763, but considering the British have kept North America in this scenario they're probably letting the colonists move west and not annoying them with proclamation lines and intolerable acts that promote the interests of what are, as far as the backcountry white savages from Paxtang and Donegal are concerned, wagon-burners and papist surrender monkeys.
