I agree. There are 43 77Es, of which (if memory serves me right) 25 are GE-powered, 18 are RR powered, and the split of bunked/non-bunked is (again from memory and subject to error) around 50-50. The 6 options on the 78X that are being actioned are, to my knowledge, done to bridge the gap vs the pre-Covid plans caused by the retirement of the 747s. This means that, broadly speaking, those 43 77Es are still to be replaced (ditto for the 77Ws, though I think the 777-9s will do some of that).
The 77Es are currently doing short-ish hops, though they also do longer haul. If we look at the GE-powered G-VIIx birds, these days they're doing short-ish hops within 4000nm from LHR. Which is also the full payload range of a 78X as per the Boeing manuals.
So, ignoring any other aspect (config, capacity of the local airports, needs) there's nothing to say that BA shouldn't replace those 25-odd GE90-powered 77Es with as many 78Xs. And I agree that the engines will be GE in my books.
Looking at the RR-powered G-YMMx, these are the routes they're currently flying, plus others they've done until recently/will be doing shortly.
As you can see, though there are some falling within the 4,000nm range of a 78X, many don't. Personally, I think we'll have a number of those replaced by 35Ks.
This doesn't mean that, in the future, we'll never see a 35K to, say, ORD or CVG (loads might require it) but it's highly likely, in my opinion, that as time goes on the 35Ks will ply those long routes, and the 78Xs will replace the 77Es on the shorter ones.
Another role for the 35K will be replacing the 77Ws on LHR-SIN-SYD. It's clear to me that those 77Ws are now the replacement of the 747 Super Hi-J, where a 380-load of Business is required but Y demand is not as big.