I was thinking about future procedural courses (assuming all units with secondary get the excemption) and I could only come up with 12-15 (wasn't 100% on some) atcu that would then require the rating.
There are currently 2 colleges in the uk that offer the rating, so with the the majority of these unit's being quite small non 24hr airports with probably 5 or so atc staff that ain't many students to go round.