If the value of the collateral has gone down hill, the banks can loose their money. As an extreme example, if guano islands were the collateral, their value is now negligible. Ot factories that are now obsolescent, or things like that.Britain can't have both problems at once. Either their collateral is still just that collateral, like a mortage on your house, where you've not defaulted yet and so don't have to worry about the bank seizing it. Or they have defaulted and their collateral is gone. But then they don't have to worry about making payments any more.
Either way, given that all the loans were secured with collateral the American Banks don't have any reason to worry. Now American Bankers themselves are probably going to have popularity problems with doing business in Britain in the future, as they'll be slandered as War Profiteers, German Catspaws, plus some choice anti-semitic slurs regardless of whether they seize the collateral or Britain has to make big re-payments while suffering under a very bad post-war economy. But the banks should not have to worry about loosing their money.
Then there's pure malice as a possibility as well, I can see an American bank taking possession of factory complex Y, and the British promptly ripping up the rails that supply it, since the line is "uneconomical," and "accidentally" sinking a few rock filled barges in any relevant canals.
Even without malice, the value can decline.
The Russian dreadnoughts aren't great ships, but the 12" guns were quite good, if small by 1917 standards. Scrap the ships, and mount the turrets where they will do the most good, or even build railroad guns. As coast defense guns, they can insure that nothing less than dreadnoughts can survive within reach, and even they will have a hard time....SNIP...
Well, the Imperatritsa Mariya Class weren't really the best dreadnoughts in the world, and it's a toss-up whether or not they're worse than the Gangut Class in use by the Baltic Fleet.
(A coast defense monitor or two could be a good use for some of them, depending on the need, or even a river monitor.)