Being a soldier, and being in the infantry are different. Basic army training is given to all soldiers. Then they get specific training for their Regiment or specialty.
The British army is comprised of - Logistics, Signals, Engineers, Artillery, Air Assault, Commando, Catering, Port and Maritime, Transport, Medical, SAS, Paras, Royal Horse, Rifles, Army Air Corp, Animal Training Regiment, Household Cavalry.... and the list goes on.
Each of these have their own specific training regime, but Catterick is the base for all Infantry training.
Jonny already had his basic soldiering skills, but needed specific Infantry training.