Here is the specific corner case that prompted this.

1. Flights on EU airline, single ticket, from far-outside-Europe to EU (flight A) and then to UK (flight B)
2. More than 14 days before, airline notifies that flight B is cancelled (no extraordinary circumstances) and reroutes onto flight C
3. Flight C arrives in the UK more than 4 hours after flight B would have
4. Let's assume that flight C arrives on time

Is compensation due under EU261 (or UK261) in this case? The obvious cancellation compensation is not, as it was >= 14 days in advance (see EU261/5(1)(c)(i)). However I've had at least one of those claims companies tell me that yes, it is due owing to the delay w.r.t. the original arrival time. Maybe there's some case law I've missed, though I could not find it on https://flightdelaypay.com/leading-cases/ .

I could ask in the airline-specific forum, but this is not an airline-specific question, and someone who had the same kind of case with a different airline and so know the answer might not be reading that forum. Having a single generic (sub)forum would address that.

Similarly, perhaps one of the airline threads already has the answer, but I have no idea how I'd find it (even if there were advanced search functionality, which there doesn't appear to be, I don't know how I'd construct a query that would find the answer and not bury it in the middle of a million other hits).