I’ve never quite understood the need for an RD cadre in the Signals (or indeed REME).There's a major difference between the Engineers and the Signals, right there - Signals SSMs and RSMs come from the RD roster, which is populated with tarmac mechanics and bull enthusiasts, who were either in trades with no supervisory qualification, or who failed to be selected for same and had to drop off the trade roster.
RE SSMs are selected from the best QMSIs, who, in turn are the best combat engineer leaders and managers.
As QMSIs they will have done a demanding course to hone their combat engineer task planning skills and learn to teach them. They will then have completed a demanding tour as an expert instructor.
RE SSMs are key combat engineering mentors for troop commanders, troop staff sergeants and section commanders.
You used to find the occasional tarmac mechanic in Plant Troop of the Support Squadron. Unlikely to have found much bull though. I’m not sure of the Corps has any pavers left.