Not sure why this is in the BA forum at all as it has nothing to do with BA.
OP chose to pay using a credit card which one presumes is a USD card issued by a US bank. He was charged whatever it is that the ticket cost in GBP converted to USD and when BA issued a refund it issued a refund of the full sum in GBP. Unfortunately the USD has apparently dropped against the GBP in the interim and thus his bank has credited whatever that amount of GBP are worth in USD today.
One trusts that if the USD had increased in value, OP would not be complaining that his currency arbitrage transaction was a success.
As others note, if there was a fee charged for the transaction (quite rare for a US-issued card), most issuers will refund that as a customer courtesy.