What happens if a squadron of County-class cruisers and Tribal-class destroyers appears behind Scharnhorst and Gneisenau during the Rawalpindi action?
Short answerThe German ships would face a very dangerous fight. The British force might take heavy losses, but the exchange could still be worth both German capital ships.
NotesThe answer turns a surface-action counterfactual into a force-position problem. The British cruisers and destroyers bring enough gunfire, torpedoes, and scouting aircraft to make escape difficult if they appear between the German ships and home. The Royal Navy would not welcome the loss of crews and ships, but could accept the exchange if both German battlecruisers were destroyed.