BA clearly has a bunch of routes where it can sell a lot of premium seats; hence the 86J 747 conversions which retain an F cabin also.

Yet they retain some 3-class 777s for routes with lower premium seat demand (and no chance of selling F).

The 787 fleet serves both these needs, in its various incarnations. You can't really put a large W/J/F cabin on a 787-8 without reducing the Y cabin so much that it becomes very dependent on premium load, but on the 787-9, it's big enough that you can have your large premium cabins and a decent economy load too.