How destroyer flotillas produced wartime flag officers

Was Britain so short of admirals that it needed to borrow one from the United States?

No. The Royal Navy promoted experienced destroyer-flotilla officers who already knew how to coordinate several ships.

Britain did not have to borrow an admiral because it lacked candidates of its own. The Royal Navy had lost good officers and initially worried about command depth, but it quickly identified captains with force-command experience. Destroyer lieutenant-commanders, commanders, and captains routinely coordinated divisions and flotillas long before cruiser officers received comparable multi-ship responsibility. That practical pipeline supplied new commodores and rear admirals and helps explain why destroyer officers became increasingly influential late in the war.