In some countries the law dictates in which currency prices must be advertised and payments taken. And it makes it easier for an airline to take payment in the local currency (or in whatever currency the fares are set for that market) for accounting purposes and for the purposes of calculating change fees, fare differences, refunds etc. If BA were to introduce such a service it would have to be a third party one (for a fee, I would imagine), but banks already provide that service anyway. Even though I myself once or twice needed to pay in a different currency than ba.com wanted me to, I think the demand for such an option is not that high.