Apparently, as I've recently found out, BA does not allow you to redeem miles for free flights on their metal. They always charge you a copay, variously mislabeled as a "fuel surcharge" or other such euphemism. Their systems further mislabels things by labeling this copay, which BA gets to keep, as "tax".

They've probably already extended this practice to all flights covered by the transatlantic joint business.