From my own experience, and if i can find it I'll quote it...
THe Army Apprentices College Harrogate brochure from the late 80's early 90's clearly states that JLs made up something like 80% of RDs, and the ATs made up a similar figure of the Supervisor Trades within the Corps.
The Princess Marina College (REME) (IIRC) made similar claims.
It is STILL pretty much correct today, as of 2009 I seem to run in to RD after RD and Supervisor after Supvisor who passed off the square of Uniacke Barracks to the College Pipes, Drums and Band playing "Auld Lang Syne" before throwing their hats in the air. Chucking their rifle and bayonet at the Junior term and heading to the Cinema for prize giving, dinner and finally.... obtaining the SQMSs signature on the clearance chit in exchange for the Black Belts and buckles (sashes and sticks for those exulted Apprentice SNCOs, SSMs and RSM).
Remarkable number of LEs seem to have come through the College too.
Re when I got to my first unit... as a Class 3 Telegraphist I certainly was better prepared and better trained than many of my Catterick Cowboy peers.
Harking back to teh Apprentice NCOs... before they were 19, these lads would have been Room NCOs, section commanders, Troop SQMS Cpls, Apprentice Tp Sgts/Cpls, Plt Sgts, Sqn Staff Sergeants, SSMs and even an RSM.
Areas and block jobs were largely supervised by teh JL NCOs and AT NCOs... and finally
In front of his/her peers, their families, the College, the Commandant (Colonol) and the Reviewing Officer (1 or 2 star normally) the Apprentice Regimental Sergeant Major would march on to the square, and take the Graduation Parade. The (Adult) RSM would march the markers and guidons on, and yell "Army Apprentices' College, Harrogate, will march on parade.... Quick March". And on would come the 6 squads of Apprentices.... Led by the Band, and the RSM at the head, who would take the parade from then on in.
The Senior Term would march off, led by the Apprentice RSM, at which point the Senior non senior term SNCO would march forward, and collapse teh parade.
Anyone else thinking that on the 22nd anniversary of the destruction of the Junior Army, the Army may suddenly think that we need to invent... I don't know some sort of circular, spoked creation.