Phew. At last !
As is well know the Great Western converted all it's broad gauge lines to standard in 1892. There have however been other gauge conversions in Great Britain since them. Please name the taest gauge conversion. Name of the line. Bonus points for date and thew two gauges concerned.
PS this does not include narrow gauge / miniature railways built on old standard gauge trackbeds.
I'll try -- the majority of the Torrington & Marland industrial (for carriage of clay) narrow-gauge line in North Devon; its route, incorporated in the Southern Railway's new Torrington -- Halwill line, opened 1925: between roughly, the new line's Dunsbear Halt; and existing SR line at Torrington. Gauges: 3 ft., replaced by 4 ft.8-and-a-half in.