Almost all carriers pay in the local currency of the passenger and have the capacity to pay in at least the currency of any country which they service. If you have a UK address, you will almost always be paid in GBP unless you specifically request another currency. Thus, on a flight from the UK to US, BA will pay in GBP, IS$, or EUR.

In my experience, carriers tend to pay at a good rate because they use the same rates for their standard commercial transactions and it does them little good to tinker around the edges of that.