The Militias were re-formed into the (Special) Reserve following the Haldane reforms in 1907 for men with no previous service as regulars. So part timers.
with regard to commissions this says it better than I can:
To gain a commission, Potential Officers have to pass through four modules of training, which together form the Army Reserve Commissioning Course.
Module A consists of basic field training and elementary military skills. This can be completed at either a UOTC over a number of weekends, or over 2 weeks at the Royal Military Academy Sandhurst (RMAS)
Module B covers training in Tactics, Leadership, Doctrine and Navigation, both in theory and in practice, with a focus on the section battle drills and the platoon combat estimate. This training can either be spread over 10 weekends at a UOTC, or 2 weeks at the RMAS.
Module C builds on the Tactics, Leadership, Doctrine and Navigation taught in Module B, with a greater focus on the theory behind these constructs. CBRN training is also added at this point, and Officer Cadets undergo a number of field exercises to test their military and leadership skills. Module C can only be undertaken at the RMAS.
Module D Once the Officer Cadet has completed their ArmyOfficer Selection Board, they can complete this final module, after which they will become commissioned officers in the British Army. Based at the RMAS, this module consists primarily of a prolonged field exercise, followed by drill training in preparation for the passing out parade.
On successful completion of Module D, the Officer Cadets receive their Commission and become Second Lieutenants. Further training that is required prior to them being considered for operational deployment and promotion to Lieutenant.
