What Royal Navy submarines could have done from Singapore

How effective might the Royal Navy's Pacific submarine squadron have been if its boats had not been diverted to Europe?

Potentially very effective, because Japanese invasion routes would have crossed several planned T-class patrol areas around Singapore.

Japanese invasion routes would have crossed three prewar T-class operating areas if the planned submarine force had remained at Singapore. A further pair of boats could have been near the landing areas. Those positions offered opportunities to attack the invasion traffic both on passage and during the landings. This assessment uses prewar operational plans, before later changes following the occupation of Vietnam. The expected first salvos are estimated at about fifty torpedoes, while the number of reloads is recalled less confidently.