POD is that the Zimmermann telegram is never sent leading to America never entering the War. Entente doesn't have anything left as collateral for new American loan. This forces the Entente to downsize their war effort. This results in a successful German offensive on France. After Entente loses France to Germany and default on it's debt to America. Britain will face financial collapse after America seizes Collaterals leading to a Landslide victory of the Labour party in the 1918 United Kingdom general election
To some extent the PoD mitigates the impact of the default as more than half the US war loans to UK occurred after US entered the war.
Also it's not good business to put your major customer out of business so a full on seizure of collateral assets is unlikely.
I'm also not convinced that the war debt is going to lead to a Labour victory - the defeat in the war will be far more important and a labout victory will mean it is much more likely that UK will repudiate its war debts to US causing a collapse in international trade. Which US will recognise so some kind of fudge is more likely.
Too big to fail applies to nations as well as banks.