Only a short idea:
Spain conquers its empire in America as in OTL. England wins the 100-year war and unites with France, and later builds its colonial empire in North America. Their colonies there have more liberties than the French ones, but not as much as in OTL, so they have a smaller population than OTL and don't rebel. Some people there, however, don't like British regiment and go to California, where the government is less efficient. Let enough time pass, and the English and French in California outnumber the Spanish and Indians. They wish for independance and rebel, asking their former motherland for help. Britain-France agrees because it wants to undermine Spain. California gets independent.