I was thinking that too, but other affiliate cards issued by Amex UK, e.g.
Nectar, do have a "member since" date.
Understood, but BA Amex cards are most definitely issued and managed by American Express UK.
Actually the BA cards are managed and issued by Bank of America, AMEX is the just the card channel, much like Visa and MC are for most. So you when you have that card you are not a American Express Card Member, thus no Member date.