The Sun Sets in California

In the days leading up to the Mexican-American War, a rumor surfaced in the US about a secret British Plot that detailed a plan for the acquisition of California. This rumor was started to hopefully spur support for the expansionist cause. However, these rumors are almost definitely fabricated. But what if they weren't? What if the British and Mexicans cut a deal regarding California. Owning California would strengthen the British claim to the Oregon territory, thus blocking the American's dream of extending "from sea to shining sea." How would a British owned California affect the coming Mexican War? What would happen if the Sun Set on California?