Why did Britain later strip forces from Malaya?

If Britain was concerned about Japan, why were forces later withdrawn from Malaya?

Because war in Europe took priority before Britain's planned 1942 rearmament was complete; a Far Eastern war beginning first would reverse that allocation.

Britain did not withdraw forces from Malaya because planners had ceased to regard Japan as dangerous. Once war began in Europe, ships, aircraft, and troops had to meet the more immediate threat there. British rearmament had aimed at readiness for a major war around 1942, so the emergency expansion was incomplete in 1939. This counterfactual gives the Far East first claim on mobilized resources, allowing Britain to confront Japan with forces that were historically drawn westward.