Since there is no time limit for the PoD besides this being the "After 1900" forum. I don't see it as that difficult to begin with. For a rough draft, go with this:
1. Starting from January the 1st 1900 a small butterfly causes Britain to work closer together with the CANZAC Dominions. Might be nothing more than some talks on a regular basis started for no bigger reason, than because some chaps who went to Oxford together are looking for a way to meet up on a regular basis and charge the tab to the foreign office.
2. When The Great War breaks out Germany goes "Russia first". UK stays neutral for the time being, while anti-Germans start working to manufacture a Casus Belli. However before they succeed the Home Rule Issue boils over, while at the same time many Britons are making money over fist selling stuff to both sides and they have friends in Parliament, too. Result is that Britain stays out altogether while being focused on Ireland.
3. Great War ends in 1917 with a CP victory, whom despite their huge casulties and general exhaustion and war fatigue APPEAR much more powerful than they really are. Meanwhile a compromise with Ireland was worked out giving them more autonomy but not independence. Some voices in Scotland start asking about a similar arrangement for them. Meanwhile the Central Powers Empire scares the UK into intensifying it's relationship with the Dominions.
4. Fast forward 15 years or so and an Imperial Federation composed of Britain, Ireland and the CANZAC is formed. Germany has recovered from The Great War, but is focused on a revanchist USSR and has no interest on getting on Britians bad side. Meanwhile the USofA never had any reason to come out of Isolationism.
A 2nd Great War may or may not happen at this point. But as long as Britain stays out it's in a comfortable position to stand as tall as the winner, especially if Decolonization is handled better than OTL. Some pieces might be invited to join the Imperial Federation, especially with much more British emigration to the colonies sans the WWs others might stay in an Alliance/Customs Union modelled after Mitteleuropa.