The Spanish colonized California in part to forestall the expansion of other empires. The territory was in danger of invasion throughout the Spanish and Mexican eras (1769-1846). Visitors and residents alike noted how easily it could be conquered; the privateer Bouchard, sailing for Argentina, demonstrated such in 1818 by sacking and burning the capital, Monterey. The states thought capable of posing a threat to the territory were Russia, England, and the United States, which eventually did the job.
Depite the presence of Fort Ross, it would have been tough for Imperial Russia to pacify and administer California. Britain on the other hand demonstrably had the resources to run overseas colonies, and already had major business interests nearby in what is now British Columbia. Around 1830, as its business connections in mainland Mexico developed, Britain appointed a consul in California, William Hartnell.
What, if any, proposals were raised in the British government or military to take California, and how far did they get?