[Zilch]

Cool scenario...I probably would have done 2 or 3 at Nelson, and another 1 or 2 at a choice tanker and take her deep to evade and listen for effect with hydrophones. I'd save the rear tube for whichever didn't sink properly (or quickly enough.) That way, as mentioned earlier by others, you'd at least slow them down enough to separate them from the convoy. You could take your pick of which deserved a torp (definitely Nelson, if both survive initial attack) and gun the tanker later if needed.

That's me...maybe not a "proper" approach but if I saw a battlewagon I couldn't just let it go. We see merchants on the seas all the time. They're officially a priority, granted, but the rarity of battleships makes them interesting targets.