In effect they did partition it. In 1957 Singapore, with its predominantly Chinese population, was kept separate from the Federation of Malaya. Keeping Singapore out helped to shift the balance of power in favour of the Malay communities. Singapore joined the Federation of Malaysia in 1963, along with Sabah and Sarawak, but it left to become an independent state in 1965.

It is possible to imagine (counterfactual history!) that the British could have chosen some other configuration of territories. If Britain had demanded the southern Malay states of Thailand as a punishment for Thailand’s wartime alliance with Japan, it’s plausible to think of a ‘Malaya’ sitting some way north of a kind of ‘super-Singapore’ encompassing Selangor and Perak. But it did not happen.