The 789s are being refit currently, its hard to say exactly when they'll be done, but mid-late 2027 seems like a reasonable guess.

That just leaves the A380s and LGW fleets. For the LGW fleets there is no timeline yet, but they'll likely never be refit and will wait until the 787-10s replace them. For the 380s they haven't started yet, and who knows how long that will take as the process is different to the other aircraft as they will have to go to MNL for their refits. But in both cases it's easy to avoid old CW once the 789s are done, just don't book a flight from LGW or on an A380, the 380 is such a uniquely sized aircraft that it basically never gets swapped around, so you'll be basically guanranteed CS from 2028 onwards if you book a flight from LHR on any other aircraft.