I think when it comes to adverts (or modelling or acting etc) it is not illegal to discriminate on the basis of sex is it? Its also one of the few areas where selecting someone according to how they look is fine.
However does seem a bit daft that the actual woman couldn't be used in the advert, given that they had already decided that she had the look they were after.
Fucked if I know what Virgin are trying to do anyway... So back in the 2009, Virgin made an advert along the lines of "look how gorgeous and sexy our female air hostesses are, fly with us!" (really???) and now to show how "progressive" they are they want to remake it, but with males performing an over the top stereotype of sexy stewardesses 
Reminds me of a piece of utter bullshit I saw on Linkedin recently (think it was Virgin again) along the lines of "You'll be surprised at what opportunities are available within our organisation to women!"
Was supposed to be all empowering, but the career they highlighted as being "open to women" was.... air stewardess! Now there's nothing wrong with that career at all, but its not exactly stereotype busting is it?