OK, to enlighten you - all ranks posted to RRF units wear the red and white RRF hackle. Other units wear hackles too, but as you rightly say everyone has their own customs, so the rules on wearing hackles will vary.
Must admit I thought that the REME hackle was a photoshop'd WAH, but it looks like there is a REME pipe band who wear them in their bonnets. Outstanding!
I was once on a course circa late 70's. Boring as hell classroom job. The bloke who sat behind me in the classroom was a fusilier. Imagine his surprise when I swung round and torched his bloody hackle with my trusty Zippo. My how we laughed!! Really lightened the course up!