Isn't there normally an announcement on board about how how flight to XXX today will take X hours/minutes?
Granted the two are a similar time but presumably there were some frequent flyers on board and I certainly (have) know(n) the expected flight time on a couple of routes. Even if they didn't say "our expected flight time to Edinburgh today will be" ...
My first thought on hearing this was EC261 as mentioned upthread. The question is how many of the pax know to claim for it , on this and the other affected flights as a result of this. Extraordinary circumstances doesn't cover BA filing the wrong flight plan, or whatever the reason turns out to be. Grounding of an entire jet type or a Icelanic volcano perhaps, flying to the wrong city for no good reason? Nope.
If BA tries to say no, I can see a class action.