Could submarines and mines have restricted Japan before Operation C?

Should the Royal Navy have used submarines and mines to close the South China Sea before the Indian Ocean raid?

Yes. A stronger mine and submarine barrier could have complicated Japanese movement before Operation C and imposed attrition without a fleet battle.

The maritime approaches could have been contested before Kido Butai reached the open Indian Ocean. Submarines and mines should have been used more heavily to close or constrict the South China Sea routes. I-30's later loss to a mine shows that such barriers could impose real costs even on long-range submarines. Sea denial offered a useful complement to Somerville's weaker fleet-in-being strategy.