United KingdomThe UK Is not a Dominion, however it is the owner of the Dominions. It is home to almost 65 million people on the mainland, and (with certain self-governing measures for said areas) millions of Africans under it's colonies. It is the leader of it's empire.
(Dominion of) Canada
The most independent of the Dominions, Canada is one of Britain's most prized colonies. Canada has a vast history under the empire. From the war of 1812 to today Canada has always been their for her motherland. Some would say Canada is secretly the British's favorite colony. Home to over 36 million people, it may not be all that populous, but it's healthcare system is the standard across the empire.
Commonwealth of Australia
Another prized Dominion, colloquially called the Britain of the south, Australia is home to nearly 25 million people, most of whom live on it's shores. Home to many poisonous species, Australia has somehow managed to get along with nature.
Dominion of New Zealand
Lots of farming, originally meant to be part of Australia. Home to indigenous Maori people. Not much exciting.
Also known as India, the Raj consists of OTL India, Pakistan and Burma. The Raj did exist in OTL, however it collapsed. In this timeline it is still around today, albeit with much internal conflict. It's home to around 1.4 billion of the empire's citizens.
Newfoundland has a fish and seafood based economy and is home to just over 500,000 people. Least populous Dominion on the planet, however a slightly larger economy then Australia's. Considered joining Canada 3 separate times, all 3 times, however, the referendum lost by a narrow margin.
Dominion of The Middle East
Formed in 1921, The Dominion of The Middle east has a lot of oil and natural resources. It is a Dominion with some self-governance, but not to the same extent as most others.