That’s my concern too, but if the USA distracts Japan, then Britain should be able to send tanks to the Caucuses reasonably quickly.
The way I play Germany, the Germans tend to have a large airforce to destroy any Allied navy in the Atlantic. If the Germans do squander their air force and don’t keep it up, then Britain would be able to use its IPCs to build a fleet to land in Norway or something.
The other thing is that this denies Germany/Italy the African IPCs, so it does help contain the Germans.