When I was in the UAS a few decades ago, the selection tests weren't quite that rigorous, but there were also interviews to determined whether you'd fit.
It absolutely is not a given.
And UAS now is not about flying so much as it's about the RAF and preparing potential officers for all branches. (Although yes, they do get flying, but far less than in my day when it was totally pilot driven.)
As organisations the UAS have never been about creating airline pilots, they've been about awareness of the RAF and creating future RAF Officers. Then again, who in their right mind would want to fly an A320 if there was a shot instead at flying a Typhoon !
G