OK, so everyone's got an opinion, but this is getting silly.

The original question specified British, so why not defer to David Baker's "Heyday of the Shotgun", which pretty clearly defines the period as from around the turn of the last century up till WWI.

Jonathan Ruffer, author of "The Big Shots" likes to go a bit earlier, but by no stretch of the imagination did the so-called 'Golden Era' pre-date the breech-loader!

I think you had it nailed down in your very first post, Ken!