Very many years ago, I went through Biggin Hill with a second choice of Navigator "just in case". I was selected as pilot but got myself chopped on the Hawk nearly 2 years later half way through the Valley course. In those days, METS was at Leeming, and there was a backlog in the multi-engine training system - the RAF was also short of Navigators. So, inspite of being recommended for Heavy Pilot from Stn Cdr down at Valley, I found myself with a letter inviting me to go to Finningley and train for a single wing - the excuse being that I'd volunteered for Nav before I'd joined.
So, beware of "second choices". I always wonder if I'd be flying in the front seat had I not played safe at Biggin.