I'm not sure that I agree with your textbooks, the US did not require that that the pound was to be devalued, they required it to return to the same convertibility as before the war. The Anglo-American loan provided $3.75 billion on the basis that the UK returned to the pre-war convertibility within a year. It wasn't until the huge capital flight after convertibility was resumed that the US would concede that the pound would have to devalue. Sourced from Globalising Capital, Eichengreen, p100-104.

The US didn't insist upon the devaluation of the pound, but instead, demanded that the pound be a fully convertible currency under the Bretton Woods post-war system. That meant that Britain had to agree to convert the face value of its money into gold or US dollars as a part of the Gold Exchange Standard. This was problematic for Britain because many of their war debts were denominated in pounds, and giving the creditors the option to convert those debts to gold/dollars placed a drain on the capital backing of the pound.

Much of the loan value was taken out of the country in this way, apparently leaving Britain worse-off, so they retaliated by devaluing the currency to keep more hard currency at home. They were also apparently angry about being charged interest on a loan from a wartime ally, but that just seems an excessive complaint.