Britain's finances were not strong enough to finance both her own effort and France's out of pocket. France was pretty much broke by 1917 and had next to no resources to fight a modern war. Though possessing a large war industry at this point, the resources to turn out artillery, munitions, and small arms all had to be brought in. This also is true for nitrates, both for munitions and farm production. On top of this food still needed to be shipped in. Capital had fled Britain, France, and Russia by this point and the only nation outside the CPs that had the wealth necessary to fund the war was the US.
Britain never funded the war out of pocket. It was always funded out of pocket. This was possible because of the strength of British credit. It's important to stress the strength of British credit: it made it cheaper to buy war material from the US than to make it in Europe.
Britain had vast overseas assets that could be offered as security, the war only cost about 14% of those. If the US had stayed neutral these remaining assets would have been demanded as security.
Britain would be in no position to refuse but neither would the US: otherwise all the existing loans would be worthless, as they were dependent on British creditworthiness.
Economically speaking the US was in for a penny, in for a pound from much earlier.