LB you are no doubt correct about YBHI. In theory it should be possible to install a new bowser that takes credit cards. I have no idea what is driving BP although I imagine it could be purely commercial - they would have to to pay a percentage of their jet and Avgas revenue to Visa and Mastercard. There may be other charges as well. There could also be all sorts of silliness involved.
Why not just sign up with Skyfuel via SAAA and they give you everything you need plus a discount?
Having said that, I always get a warm feeling at YBHI taking my two jerricans to the servo by the roundabout near the airport and filling up with UL95 for about a dollar a litre less than Avgas.