Just as a data point I tried to make a BAH booking for flights plus car for Mr NW in Club as he needs the double TPs.

I also tried to make a flight only booking for myself and the junior NWs in Y.

I tried my BA Amex, my BA Amex on Mr NW’s account and Mr NW’s BA Amex. All failed multiple times. Sometimes the system told me I still needed to input the 4 digit code and kept taking me back to the payment page. Other times it said card declined and told me to contact the issuer which I did to no avail.