I was more thinking in general terms to avoid unnecessary complications with introducing another differential, bit I get your point that they only need to enable such for a few specific classes which would reduce the complication anywayThere isn't a requirement for 220s (or 222s/180s/HSTs) to run on the WCML at 125mph.
Avanti (and TPE north of Preston) are the only users for the forseeable future.
Non-pressure-sealed stock (eg 350s) will not be operating above 110mph, nor will 730s.
221s will be gone.
NR really only has to deal with 80x and 397, and the future HS2 stock when it is known.