According to the Wikipedia entry on the Night Ferry, they did consider keeping some Mark 1 sleepers and using them as a replacement for the Type Fs, although I would guess that that was only after the 1970s Channel Tunnel project was abandoned. That would have kept the Night Ferry going for a few more years at least, but they would have needed a number of modifications to enable them to run on SNCF and SNCB metals including fitting retractable steps for the lower platforms in France and Belgium.
I suppose that might have happened if it weren't for the 1978 Taunton sleeper train fire, but sooner or later they'd have come up against the deadline for getting rid of stock that had asbestos. Also, I don't know if they would ever have been allowed to run through the Tunnel without Central Door Locking and with non-retention toilets (and with each compartment having a chamber pot that emptied onto the track when you put it away), although I believe that some Eastern Region Mark 1 sleepers latterly had a kind of controlled emission toilet system that only deposited liquid onto the track. Even that probably wouldn't have been considered acceptable in the Tunnel, though.