I believe the oldest 77Es at LHR are being replaced 1-for-1 with the 777-9s. There are 18 GE 77Es at LHR and 18 777-9s on order. This doesnt mean the 777-9 will fly the same routes as the 77Es as BA will mix it all up and put the best planes on the routes. I’d expect the 77Ws and 787-10s to replace the 77E routes, with 777-9s doing some longer stuff.

That still leaves 25 77Es, all 3-class some at LHR and the rest at LGW, to be replaced. I’d imagine the 787-10IGW and 350Ks to be top of the list for this job, perhaps a mix of both.