Jack -- LoB is done on an individual basis, whereas tactical reserves are constituted from formed units or sub-units.
Recalling that Jack's original question was about Paras, it might be as well to recall that in WW2 the airborne forces constitute a slightly different kind of weirdness from the general weirdness of the British Army overall.
The parachute battalions (the first of which was formed originally from an Army Commando, briefly named 11 SAS) were formed from volunteers transferred from other regiments. This was true of a number of British WW2 elite units, including the Army (though not the Royal Marine) Commandos, and all the wonderfully eccentric little private armies that abounded especially in the Mediterranean theatre such as the SAS, SBS, COPPs, LRDG, PPA, RSR (the Calcutta Light Horse famously provided a special forces detachment all its own, but that was Indian Army, so a different flavour of eccentricity). The only source for parachute-trained replacements is Depot Para at Ringway (later Aldershot). Having thier best junior leaders creamed off by recruitment to organisations like the Paras must have irritated a large number of COs, and the severest punishment in the units themselves was to be RTU'd (Returned To Unit).
Airlanding battalions, on the other hand, were converted wholesale from line infantry battalions (as RM Commandos were initially formed by converting Royal Marine Light Infantry battalions).
Then there's the Glider Pilot Regiment, if you want lots of NCOs…
All the best,
John.
