That does not make any sense whatsoever; regardless of flucation, you should still get whatever your base price is.
In you case it's a UK customer so the flucation does not even apply. Th eguest might get more for their money due to GBP drop but your base rate is set and must be paid by anyone booking. If Airbnb have held the money for a while and as a result of deflation the value has dropped,that can't be bassed to you. Liekwise if the GBP had inflated, ypu would still only get your base rate.
I'm glad you got it sorted and thank you for drawing attention to this hiccup, could easiy be missed but now we know to keep an eye open.
BW, Ama