I've also had experiences where the flight times changed on the day of flight. Sometimes I received a notification of it, other times not. But I would certainly never depend on the time on a printed ticket, or even a mobile boarding pass, to be accurate. I would use another source (updated in real time) to confirm the departure time.

Does EU compensation really apply here? I wouldn't expect this to be treated as IDB. OP was not there at T-10.

I believe you could look at the history of that particular flight on some online tool (flightaware?) to see when the changes to departure time were made. But isn't there guidance from all airlines that you need to be at the gate roughly 30m before the scheduled departure time of your flight?

I think that even if you can find an understanding soul @ BA that agrees it was unreasonable to expect you to be aware of the change, it'll be tough to get that person to approve £275 in compensation.