its all changed within the last 18months or so, now you'll join as an Aircraft Maintenance Mechanic, where I think you'll undergo approx 6 months of "trade appreciation" where you do a bit of all trades also do all the flight line servicings learn about handling aircraft etc, you'l then be posted to a sqn where you'l work the flight line for about 2 and a half years before being sent back to happy cosford for a Further Training course where you'l do your original choice of Avionics which will last a good 12/13 months.
Its probably better how the training works now as when went through 3 and a bit years ago i was at cosford for 14months did all my basic trade training and passed out as a propulsion technician then we had to do an NVQ (so you get promoted from SAC to SAC (T)) by carrying out certain tasks which took about a year to do and i've just recently been at cosford doing my multi skilling course to airframes so now im an aircraft technician mechanical (still a propulsion tech at heart tho!!)
but when you come out of your final trade training, i dont think you do an nvq plus for avionics it now incorporates electrics as they use to be separate trades, so no further multi skilling course which is good! keeps you away from cosford!!
hope this is helpful!