It was indeed Junior Jet Club and I was a member myself in the late 1970s (yes, it existed post-BOAC). You were given a log book which you could present to a stewardess on your flight, who would take it to the flight deck where the captain would fill it in with the details of the flight, the aircraft registration and so on. I'm sorry to say I think I've lost my log book but someone here might be able to scan one?