Prior flying experience Vs non is certainly taken into account when deciding upon how EFT students have performed. You would expect an ex-UAS member to be excellent, or someone with significant other prior experience, and their performance and future potential would be gauged accordingly.
Of the three streamed FJ from my EFT, 1 had UAS experience, one had zero experience, and then there was me - I'm certain the chap with zero prior experience was picked before me, and rightly so.
Basically none of the above routes either advantage or disadvantage you when it comes to a streaming board. My personal view is that having flown before was an advantage to me and me alone - at the streaming board though, I would probably have been compared to people with zero experience who performed to the same standard and they would have been rightly picked before me.
If someone is in a position to learn to fly before they attempt to join the RAF, why shouldn't they? I've wanted to fly since I was a very small boy and I was lucky enough that I could do that before I joined the RAF, although joining the RAF was always the goal.
What I'm trying to say is that flying is great in whatever form you do it, whether it's off your own back, or starting form scratch in the military - either way, if you make it to an EFT course, you will be judged on your ability and potential fairly at that stage, taking your prior experience, or lack of, into account. The rest is irrelevant.