The Washington Treaty forbade the British Empire from building any fortifications east of the 110 East meridian apart from the coast or Australia, Canada and New Zealand. That leaves out the likes of Hong Kong, PNG, Rabual or Fiji and basically makes Singapore the most forward place a naval base can be built. Similar conditions applied to the US, Hawaii, Philippines, Guam and other Pacific Islands being off limits. Apparently this was an important condition for the Japanese to accept the inferiority in ships and was negotiated in 1920-21, 20 years before the Pacific war.
As for why you'd want to counter Japan by building a naval base in the Indian Ocean, I have no idea.
As for why you'd want to counter Japan by building a naval base in the Indian Ocean, I have no idea.