Why submarines were scarce for Singapore

Could Royal Navy submarines have been stationed in the Gulf of Thailand before the Japanese invasion?

Only in limited numbers: many boats from the Indian Ocean and China Station had already been reassigned to the Mediterranean.

Recalled merchant traffic and warning of Japanese hostilities could have prompted a submarine ambush in the Gulf of Thailand. The Royal Navy would have tried to use whatever boats were available. Many submarines formerly assigned to the Indian Ocean and China Station had already been withdrawn for the intense Mediterranean campaign. Pressure in one theatre had therefore removed options from another before the local crisis began.