Are these the ones with a 'square' edged ferrule (like Cinelli) engraved GB on side and 'Fabrique en Angleterre' on the other? If so I've got a pair which are also engraved 26mm whereas most GB bars are 25.4.
Regarding the slipped ferrule - no idea, sorry! How about tightening them tight - but not fully tight - and see if the 'inner' part of the bar will slip back round and then fully tighten them?
Just a thought so if it don't work - I ain't responsible guv.
Today is the yesterday of tomorrow.