the russian sub is useful for the simple reason that you can attack the german cruiser if it ventures to attack the the UK destroyer on G1. after that, it will be useful on a case by case basis.
if the german cruiser fails to attack the destroyer on G1, then the UK has gained by implication, and the sub can perhaps attack the cruiser on R2 if the UK can’t (or chooses not to) attack the germany navy on UK 1.
interestingly, an attacking sub versus a crusier is a 50-50 battle, despite the 2 to 3 difference in combat values, due to the sneak attack capability.