I don't think you should read too much into BA's use of a 744 yesterday. It's by no means the first time they've covered short haul routes in this way. It's just a sensible response to operational disruption, and (as Shed says) long may it last.
And the "Union" distinction between long haul and short haul cabin crew is largely history. The MAN-LHR route is operated by "mixed fleet" cabin crew who are licensed to operate on multiple aircraft types, including long haul aircraft such as 744s and 777s.