I'll be honest, my first reaction to hearing about what hit the Earth, was thinking about an Alt History book called Peshawar Lancers by SM Stirling. Not everything matched up, as Lancers was only set around the late 21st century by my recollection, with the impact having occurred in the mid 18th Century as I recall. And Stirling went and patched on a supernatural enemy part way through the story for some reason. A mostly interesting book of the aftermath of a orbital strike in the mid-Industrial Age & overall I will recommend it to those of you interested in that type of thing.
Based on what you briefly provided about the history of this alt Earth, it seems to have followed a few similar paths (UK, African Empires, mostly desolated North America), but with more recovery time and a chance to settle into a new normal as well. I will look forward to seeing how this story continues, cause while I have definitely been enjoying it, this new Earth has absolutely made me perk up in interest.