(for example, when Friends, with which Coupling obviously has a lot in common, became very popular, there was seldom a week that would go by without someone in the popular press commenting on Jennifer Aniston's hair)

Ah-ha. That explains the whole thing. Coupling is a deliberate Friends clone, right down to the characters: One of the female leads has to have funky hair. Somewhere, there's a very strict "sitcom formula" written down, and that's one of the rules.