I suspect one of the killers for Couchettes in UK stock is the lack of space.
You're not going to fit a 6-berth couchette in the height available and a 4 berth couchette is basically 2, 2-berth sleeper cabins without the wall and without the mattress.
Of course building a new rail system and sleepers to Continental gauge would allow for significant passenger comfort improvements, even on day trains, but so far the only planned line to that gauge is stopping at Crewe...