Well, I mean, not necessarily obvious to everyone, but they were British and SF geeks, so it seemed like something they'd know.

On reflection, I think I was being unfair and they're probably too young to have come by the relevant context naturally. Not everybody my age knows as much about 1960s-70s British telefantasy as I do, even if they have the advantage of being British. :)

(The answer is that Cathy Gale, Emma Peel, and Tara King were the three best-known of John Steed's sidekicks on The Avengers.)