Ldnn1

I don't think you can. Ultimately this is the risk you take with a tight ex-EU itinerary.

EU261 is designed as an all or nothing scheme - where it applies there is generally no 'my delay doesn't qualify so I'll claim my delay losses outside it' route.

Edit to add: The new Consumer Rights Act is due to come fully into force in October for air transport so in future that may offer an alternative redress route in some situations, but it won't help you now.

You could anyway complain to BA in the hope of some avios comp for poor service but I wouldn't hope for much. You can also claim the original routing credit as Pilot37 notes below - that should be a simple one.