If you consider the issues raised here, recent IAG statements and decisions and the state of the two suppliers so suspect BA will want to split the risks it faces by placing Airbus and Boeing orders. I think they will also want to add resilience to the fleet and have scope for modest fleet expansion.

I would therefore not be surprised to see an order for 12 Boeing 787-10 with GE engines and the slightly increased range. Such aircraft could serve nearly all LHR long-haul routes with BA retaining the option to
allocafe 777x, 787-9 or A350-1000 aircraft to longer routes which shouldn’t be ignored but are not a significant proportion of the operation.

The 18 777x will add significant capacity when they arrive and preserve First class. BA would have to determine if they wanted to keep a common four class layout on 787-10 or specify the new batch to be J P Y only with more economy seats creating an additional subfleet at Heathrow but replicating the 777 fleet at LHR with no First class. Less sub-types at LHR would be good but BA commercial types may not want more first class capacity?

in order to spread risk I’d then expect BA to acquire, say, 15 Airbus A350s for LGW which gives scope for growth at LGW. There is some debate about whether A350-1000 can be accommodated at LGW but I think the 9 abreast A350 seating vs 10 on a 777 means they would want a -1000 version to replicate capacity.

BA might take the view that the 777x capacity increase might permit some 777-9s to be ordered as 777-200 replacements?

Watch out for curved balls like second hand A380s, moving 787-8s to MAN to reinstate long hail routes or even A321XLRs and 777-x order.