>I wouldn't be at all surprised to
>learn, however, that some British riflemen made a habit of scavenging
>extra magazines whenever possible and stashing them away for a rainy
>day, because what a pain in the ass.

For a value of 'some' that boiled down to 'pretty much all'. Some preferred the extra ammunition you could carry if you went with clips but most wanted at least a few quick changes.

> This is mildly ironic given that cavalry were pretty
>conclusively shown to be of no military value in WWI,

The men on both sides of the Kaiserschlacht in 1918 would be surprised by that. The main reason it failed was that almost all of the German cavalry was in the east and it was stopped by the ability of British and Canadian cavalry to quickly deploy.

However, it is true that there was no longer much of a need to have the full anti-cavalry bayonet: By WWI cavalry forces generally dismounted to fight, the horses were primarily for redeployment. Men were still trained for lance and sabre work in case it came up, but it was expected to only occur in the pursuit phase.