Indeed aware of that loan. "Virtual indebtedness" in the sense that the amount was relatively small to the UK when it was eventually repaid, the terms of repayment were very generous, and the repayment itself was very much the fulfillment of a legal requirement, and not something anyone would think the US would have pressed if more time had been requested.
The interesting thing to consider is that although the repayment terms were generous given the general context of the Marshall Plan, the US should probably have had to go an extra mile given the UK's contribution of its Tube Alloys program [0]. This nascent R&D project was moved over to the US, expanded and upgraded - and nowadays we call it the Manhattan Project.
[0] https://en.wikipedia.org/wiki/Tube_Alloys