The war revealed that we have two really conflicting needs for our ground forces: territorial defence and colonial service. The obvious solution is to create two organizations, each focused exclusively on one role. Consolidate the Militia, Volunteer Force and the Imperial Yeomanry into a single garrison force for the UK itself, and establish a training regime that allows these units to serve as a training cadre of reservists.
The main Army will continue to serve colonial and imperial needs, and thus needs to be flexible. Increase joint planning cooperation with the Dominions to share the economic burden better. Stabilize the budget so that there will be less need for drastic wartime spending, and so that reforms can be concluded in a rational fashion. Get the political parties involved and committed to imperial defence, and improve the public image of the Army to promote the need for funding to the voters.
Establish more local garrisons to potential flashpoints and areas of unrest, so that we'll have senior officers with good knowledge of the local conditions at hand in all potential threat zones in Africa and Asia.
As for infantry doctrine, rapid and accurate rifle fire from dispersed formations and entrenched earthworks is clearly the way for the future of infantry. NCO training, camouflage, spade work and marksmanship are to be emphasized in the reformed future training regime. Conduct live-fire tests for the maximum range our infantry fire will be effective, and estimate the best possible future cartridge for our infantry from this analysis. Increase the number of machine guns, as they serve as excellent area denial and garrison weapons.
The artillery will need to refocus on long-range fires with heavier-caliber pieces. Once again difference for colonial needs of mobility and flexibility and the garrison needs for firepower will have to be considered.
Cavalry will have to abandon her traditional role as a shock force, and instead emphasis mobility as mounted riflemen. Armored cars will make a fine addition to her traditional scouting role.