I don't think we've had much discussion about foreign exchange conversions, and this post may be stating the obvious, but I just bought my first ex-US itinerary directly with BA (125 stock) in quite a while, and at purchase BA specifically told me the exact number of TP I would earn, including cabin bonuses. This number correctly correlated with the fares and surcharges converted from USD to GBP, plus the TP bonus for booking BA codes, and implies that BA convert from USD to GBP at time of payment (as opposed to when the ticket is flown and the revenue is booked).

In contrast, my AA flights (purchased in USD and only reported to BA once flown) seem to convert using some sort of periodically adjusted fixed rate. My first thought was IATA FDR or MMR, but my flights from mid-May to end of August 2025 all converted at exactly 0.79289 USD to GBP, while those in mid April through mid May converted at 0.79983, and then 0.75226 before that. There's no correlation to when the tickets were purchased.