Well Singapore was down by the middle of February, so they had nowhere to base from. ITTL that's not an issue.
For your information :
British Submarine Operations in the Pacific
By Arnold Blumberg After the Royal Navy’s traumatic departure from the Pacific Ocean in early 1942, the 4th Submarine Flotilla and its depot ship, the HMS Adamant, operated with the Eastern Fleet based at Trincomalee––a large, natural harbor located on the coast of Sri Lanka in the heart of the...warfarehistorynetwork.com
Again, which submarines? The link provided by @Admiral Bloonbeard admits that significant RN submarine reinforcements were only send after the Italian armistice in 1943. Before that the wiki about the Eastern Fleet only mentions HMS Truance as the sole British submarine.