Interesting question!
I guess, its a bunch of reasons for this matter!
British goverment was long in colony business and settled early in the south african cape, which was the stepdoor for all early big game hunting! And Everything needed out there, Great Britain delivered!
And the British had very ambitioned gunsmiths! A look into „cartridges of the world“ shows the abudance of british cartridges most of them obsolete today!
The old days must have been an interesting time. The british gunsmithes made not only guns, they also made the ammo for those guns! That was essentiell for especially double rifles. Everybody who ownes one knows, how important the right ammo is for those double barreled rifles! (Once soldered together, You better stick to the assoziated ammunition!)
This gun and ammo combi offered all oportunities to gun craftsman to be independent in their own creations and developments!
Another reason might have been the ego british gentlemen who were very much in hunting sport! Holland & Holland, Rigby, Jeffrey, Westley & Richards, Purdey and so many others had to offer new developments to the british wealth and high society and as a mans thing, every cartridge had to be bigger than the other one! I think, all those today still wellknown british companies supported this trend with high grade big bore rifles and doubles and made a fortune out of this run! One had to wait years to get his gun built!
If You read „Pondoro“, its surprising how unusual he ordered himself guns and ammo!
If I remember reading right, he did by letter!
And settled finally not on a 450, 470, 500, 505 or 600, no he chose the mild 450/400 NE Jeffrey as the allaround-doing-well-for-him- cartridge!
Which brings me personal to the fazit, that all those different british cartridges weren‘t nessesary actually, which the Germans did realize and therefore were done with Otto Bock, by inventing the 9,3x62 with standard and easy available shell fitting in standard Mauser k98 action with in comparison mild recoil and Softpoint bullets, finishing all a landowner in Africa could have ever been confrontaded with!
They might have kept it my way:
Keep it easy!