The US fully commits to the Central Powers side somehow. USN suffers at first but it outbuilds the Royal Navy later and starts to act in the Caribbean, Mediterranean and Asia. Japan and Italy switch sides, Egypt is invaded and the Ottomans are given a second wind in the Middle East. Then somehow, a hardline government takes over the UK, decided to turn the war and refuses to surrender. India enters in full revolt and the remaining dominions declare neutrality.
Russia is defeated as OTL. France asks for an armistice. Then everyone seeks and destroys the RN remainders and the UK only surrenders when the first German and American marines land on the island.
Result: The UK loses eveything, colonies are taken by US, Germany, Japan, Italy and Ottomans. India and Egypt become independent, dominions become republics, Ireland is fully liberated as a republic, Falklands go to Argentina, even Siam is offered a few pieces. Then years after the treaty, Scotland secedes.
The trick if to find a POD that commits the US into really hating the UK that much, and with a 1900 POD is difficult, given that the entire UK policy for the US was about befriending it. Difficult but I'm sure it's not impossible.