Perhaps the single most common British double you'll encounter would be the Webley & Scott 700 series, which appeared right after WWII. (700, 702, 701, etc.) Most of them are 2 3/4". And the 720's--a 702 in 20ga made expressly for the US market--were 3" guns. (At under 6 pounds weight . . . I'll let someone else shoot magnums!) Early model 700 12's were 2 1/2", but most of them you'll encounter will be 2 3/4".
And like Le Fusil, I had a very lightweight G.E. Lewis from the 30's (right at 6#), that was delivered with 2 3/4" chambers, 1 1/4 oz proof.
