We only finished paying back the Americans for WWII loans as recently as 200x
Newsnight triumphantly announced this fact, leaving me with the mistaken impression that UK was utterly debt free at a time when Brown was still Chancellor.
Not so. Just that this particular debt item was closed on the books.
Unfortunately, the graph you pointed to does not seem to distinguish between the US war loans and ordinary government borrowing.
I guess we got out of that problem because post-war was an economic boom time, thanks to all the wrecked things needing to be rebuilt. "War is good for business" as the apocryphal saying has it.