What I mean by this is the naming convention that pre-dates the Napoleonic Wars( I think) that result in regiments like "Oxford and Buckinghamshire Light Infantry", "South Wales Borderers" , and so on and so on. I imagine a part of this was the old British tradition of army expansion and contraction and budget restraints between wars. But it just seems a bad net result on the whole. People less aware of local regiments, potentially fewer joining and a break down of tradition, and a loss of regimental esprit de corps that exists with older formations.
At the risk of being contentious, the British Army didn't so much abandon regional regimental affiliation within the Regimental System as much as Germany took said part of the British Regimental System out into a paddock, forced it to stand in a field and shot large holes in it. Twice.
That said, the system was never completely abandoned.
I'm on holidays in Malaysia visiting family for CNY, so I'm away from most of my materials/sources.
As much as the introduction of the county system in the relative peacetime of the late 1800s that replaced numbered regiments proved disruptive to the system and army in many corners, the conflagration provided by two World Wars (and the exhaustive need for Britain to throw her entire Empire at both of them) really dealt county regimental identities a massive blow both directly and indirectly.
Directly (certainly from a WW1 perspective) as battlefield requirements meant a massive expansion of all regiments, as much as the process of industrialised combat, industrialised attrition and industrialised reinforcement completely rewrote the rules by which the system was meant to work.
In/Famous "Pals" battalions aside, post mid 1916 it was more than possible to sign up under the Cap badge of choice and have that mean nothing once you hit Etaples where you could be reassigned to a different regiment at the Army's whim. Likewise, it was possible and not atypical for soldiers returning to duty after convalescing from wounds to be reassigned to a different regiment, either (in extreme cases) because the their original unit had been attrited out of existence, or (more commonly) because the unanticipated needs of the Army to prosecute a war the Regimental System had not been designed to service overrode regimental sentimentality and personal preference.
Indirectly as post-1945, the Army struggled under numerous structural reviews and force reductions brought on by Britain's economic exhaustion and dismantling of it's Empire, meant wholesale disbanding of some regiments and amalgamation of others.
Within these constraints, the Army has attempted to carry on regimental identities where possible. The somewhat consolidated identities these regiments take up to our 20 year cutoff may seem irreconcilably disconnected from that of the past, but it's probably worthwhile remembering the "good old days" weren't that good...
The South Wales Borderers of Rorke's Drift fame, after all, contained relatively very few Southern Welshmen, and certainly weren't as prone to bursting into spontaneous song as Hollywood would have us believe.
I'll have spotty internet for the next few days but will definitely get to any follow up questions next weekend.
For now, two of Richard Holmes' books, Redcoat and Tommy would be my recommendation on accessible starting points on the topic.