Nephoi

i dont see any problem here.

you are eligible for a full refund (X GBP). you got the full refund: X GBP.

you bought something in GBP, you got the GBP back. if you dont want the exchange rate risk, dont buy something in foreign currencies.
Sometimes you "win", sometimes you "lose". i kinda have the feeling that you wouldnt have opened a thread in the case of earning 60 bucks due to changed exchange rates.

for the record: getting Y dollars back today because you paid something with GBP earlier this year, isnt a full refund with same payment to me. Its not British Airways fault/error/issue that you dont have a GBP credit card, if you have to buy the GBP first, thats your issue. and sometimes you even have to pay a fee for that. thats the same problem I have when I buy domestic tickets with AA. sometimes i lose when i refund them, sometimes I win.