So Britain defaulted against its loan from the U.S. to pay for WW1 in1934.

When Britain went back to the US in June,1940 to seek assistance, the U.S. did not extend a line of credit.

Britain therefore had to pay for arms and supplies in gold or money (negotiable security). When this ran out, America signed the "Lend Lease agreement" which Britain finally paid off in full in 2006.

At peak the cost of the war to Britain was estimated at 60% of its GDP.