I thought it was a really good choice. It could've been Sir Steve Redgrave or Sir Chris Hoy, or Dame Kelly Holmes (did you notice the announcers didn't refer to anyone as Sir or Dame?), and it would've been fine, but it would have been predictable.
Since "inspire a generation" is the motto of these games and there's been so much emphasis on leaving a legacy, it made perfect sense to have some unsung athletes from the next generation light the flame.