You're both pretty much getting to the same answer, but Scroggs has the cold hard truth of it.
Fact is, while they do note your preference when it comes to streaming, that can have next to no effect on where you end up. The RAF will place you just where IT neeeds and IT wants you regardless of what you want.
ZH, you're friends got lucky. They asked for fastjet and they happened to be streamed for fastjet. Thats just luck of the draw more than anything and is not an indication of any new procedures that allow new recruits to choose what stream they want.
Your example of the ATCO is a very different situation. Comparing someone who's been around for a bit, has built up some experience/contacts/life in the RAF to new recruits who all want to fly fastjets but can't.
NB: I'm not RAF so feel free to disagree with whatever I've said! I did look into the whole process and talk to people a lot though before eyesight problems ruled me out of joining as a pilot.