The 787 and A350 are really quite different, as mentioned above and many larger airlines have ordered and operate both. For example the 787-10 and A350 are similar in capacity, their capability is very different. The 787-10 is ideal for the shorter US routes - remember BA will stiff both the cargo and passenger decks. This shortens published rage quite dramatically.

BA will undeniably have a fleet mix, but it has a complex route network, so flexibility isn’t a bad thing.
787-8/9 - Long/Thin/New routes to The America’s and Asia (there’s loads already!)
772 - Flexibly used across the network, where cheap ownership costs matter more than higher fuel burn. Likely leisure routes, Near America’s, Middle East and Gatwick.
787-10 - High frequency routes with large volumes, NYC/BOS etc
A351/77W High Volume Routes to Asia and Australia
A380 - Super High Volume/Premium heavy routes.

Knowing BA as we do, if they could have saved £5 by NOT adding an additional fleet type, they would. But the overall sums add up. The long term 777/787/350/380 isn’t wild.