Per Wikipedia, Monty was only a temporary lieutenant-general, and only achieved that rank in July 1941, having been promoted to major-general in 1938.
Does we have the necessary rank to become ground or overall commander of British forces in Malaya? We'd want him in place soon after the decision to send the Canadian reinforcements was made in January 1941, as this occurrence would demonstrate the new importance Britain placed on protecting Malaya via offensive capability.
There's nothing wrong with Phillips, and you're not going to send Sommerville to command two capital ships and four old destroyers. Maybe later once Indomitable, Hermes, and the ABDA forces are in place. I've often wondered why Phillips didn't use his eight Shagbats to reconnoiter up the coast instead of sending his capital ships within striking range of IJA land based airpower to search for the enemy landing craft, but there must be reasons. Apparently Phillips was a capable commander, well missed by the RN.