Ah, fantastic - first evidence I've seen of three ringers (should have clarified - 2 1/2 ringers seem the most common, but 2 with two half rings inbetween - ATA captain style - I have not seen), though it shews the wavy R/O lace quite well(third pic, on the right). What is very interesting, and had no inkling of beforehand, is the women (well, I think it is!) in the background of the third pic (I'd only come across the boat crews). Are the KD clad ladies ATA or BOAC, do you know? If the latter...well...very interesting indeed.
You're quite correct that BOAC didn't train any new pilots, but the information I have certainly suggest that new navigators may have been trained by BOAC (primarily ground based, it appears - again, the information as I'm sure you understand is unyieldingly fuzzy and the focus of most that is written is on the pilots. The chap on the right in the second pic is definately a nav)