In this timeline, the Mexicans sell the California Territories (Baja and Alta) to the United Kingdom in the Mexican-American war. Sort of an "If I can't have it, neither can you" type idea. Additionally, the British also have control over the Oregon county.
As I was developing this concept, I began to wonder, would we see the area of The Oregon county (including OTL British Columbia) join an alternate Canada, or join British California?