From Hugh Longworths "Pre-nationalisation coaching stock" it doesn't look like any Bulleid passenger coaches lasted after the end of 1968.
When the Southern converted loco hauled Mk1s to the VEP/TC sets sequences of running numbers were converted (presumably to get absolutely-identical batches). Many of the target coaches were allocated to other Regions, and conversely the ones already on the S.R. weren't in the series, so there was a lot of inter-regional swaps. It seems that the Southern didn't have enough Mk 1s to transfer, so that some Bulleids were also transferred. They were repainted maroon, possibly before transfer. As they weren't repainted maroon until transferred it's possible that this was after the change to B&G but possibly dons as not to draw attention to them.
As a result, in the July of 1968 I rode on SC14xxS (poss 1453) in maroon from Glasgow to Fort William, hauled by an NBL Type 2 (I was despondent that I had narrowly missed the named K2s, though). I think I saw one on the East Coast, too. Presumably they were really stop-gaps, but a bit more physical than the usual 'paper transfers'.
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About the same time, I saw a green Mk2 FO in a Deltic-hauled train at Newcastle presumably after transfer as surplus by the S.R. and this would have been built after the changeover, but possibly in the old livery to avoid a clash with the rest of the S.R. fleet.
I recollect vaguely that T.P.O. stock also received B&G - the Maunsell one on the Nene Valley still has traces under the preservation green. I dimly remember some of the M30nnnM series being in B&G but some were built comparatively long after Nationalisation.