Why would the British not continue using the same line infantry model they had been using in colonial warfare to date?
Where would the rebels in the colonies be getting the advanced weaponry that would justify a major sea change in how counter insurgencies are fought?
We’re talking about a world with no WW2 and where the Soviets are falling apart. There’s no mass of WW2 surplus guns or SKS/AK-47’s to funnel to rebels. Rebels are going to be armed with bolt action rifles.
You actually proved my point.
OP indicates WW2 never happened so no Panzer Armies racing across the Soviet Steppes. However plan 1919 had been envisaged but never implemented. The Lieutenants in the trenches in 1918 by 1950’s would be in the most senior positions at the MOD. The current instructors at Sandhurst would’ve been themselves taught by men who served in WW1.
Conflicts in the 1920’s & 30’s involving the British Army were irregular warfare battles. The Irish War of Independence, involved assassinations of ‘Key figures’ therefore procedures and skills would need to be learned to prevent this form of attack. The Imperial establishment wouldn’t like their game of cricket to be disturbed and heaven forbid if afternoon tea was interrupted. Then there would be her ladyship’s game of Crochet on the lawn. This is SAS type operations. Enemy commanders would themselves become prime targets for the Crown Forces. Again this is SAS type ops.
Flying columns would be the prime movers, leading to ambushes, leading to skills to ambush the ambushes, raiding enemy compounds etc; this is commando type ops.
In the Palestinian conflict people were tied to the fronts of trains to prevent ambushes. The 2 pounder pom-pom gun was used to lay down covering fire. Most likely this vehicle mounted gun would be the heaviest weapon in the arsenal, similar to the 40mm grenade launchers used today. Again instructors at Sandhurst would be more familiar with this type of warfare so more innovative commanders, Wingate, Sterling, Maynewould become more prominent.
As by mid 50’s both the US & BE would have nuclear weapons. Whilst these may be few in number, and never been used in combat, the rational for large set battles, Waterloo, Somme etc would be diminished, a nuclear detonation would, ‘in theory in OP’s timeline’ wipe out large battlefield armies in minutes, rather than days/weeks of WW1.
Britain would still be using the good old .303 as per otl there would still be millions of rounds still lying about.
Automatic rifles were around ie Colt Monitor but these would be expensive and no readily available to either military or insurgents.