Sonora
I guess your point of Departure needs to be around 1750. First Portugal, then France and then Spain turned on the Jesuits (who founded Los Angeles). In OTL, the Society of Jesus was saved from extinction by Frederick the Great of Prussia and Catherine the Great of Russia. It's ironic that neither was a Catholic.
Suppose the protector of the Jesuits had not been Frederick but King George of England?
In 1758 there was an attempt on the life of Joseph I of Portugal. The Marquis of Pombal was able to use this attempt to move against his rivals, the Tavora family, accusing them of conspiring with the Jesuits and the British. He also used this opportunity to order the Guarani tribes in Paraguay moved from their ancestral homes. When they revolted, he used that as yet another stick with which to beat the Jesuits.
In OTL, Pombal was able to persuade the Spanish to expel the Jesuits from Spanish America in 1767. Pope Clement XIII spent most of his reign trying to save the Jesuits...
In this other timeline, Clement, realising the growing power of Jansenism in France was affecting the whole of the Catholic world, turned to Britain.
As luck would have it, Britain had heard wind of an unholy alliance between France and Sweden to invade England and to place Charles Eduard Stewart on the throne (in OTL the French attempted to form such an alliance but the Swedes rejected it... in this timeline, that doesn't change but the British are worried that it is a possibility).
Pitt took Clement's suggestion as an opportunity to end the Jacobite threat forever by encouraging a Catholic Settlement in the Jesuit held area of California near the border with Russian America (not formally a part of Spanish America). Britain agreed to invest Sonora as a British colony to be run on democratic lines with the Roman Catholic Church, rather than the Cof E being the established Church.
The British were able to persuade many Irish and Scottish Catholics to join the venture. This was helped no doubt, by the (true) rumour being spread that "Bonnie Prince Charlie" had converted to Protestantism (it was true in our timeline as well and was a major factor in the ending of support for the Stewart Dynasty in Britain and France).
How's that?