I seriously doubt it'll ever approach a threat. The real problem is even if the British and Mexicans somehow swept the Atlantic fleet from the waves any supply route to Mexico is well within range of US based airpower. Basically even in a best case naval scenario any and all convoy's will be threatened. I honestly don't know what the British are thinking in doing anything that could antagonize the US. Even they should know at this point it's basically impossible for them to wage a war against the US. What the hell would the British aims even be? Any territorial changes on the mainland would require direct invasion of the US and unless you get a United South America no nation is a threat to do that.