Yesterday I had my third flight in a row delayed due to BA's ridiculous and totally non-enforced cabin baggage policy. Not helped by a complete lack of assistance by the ground staff and the CC seemingly not feeling empowered to do anything - or maybe just CBA?.....
....I flew Easyjet from the same airport 4 weeks ago and their policy on size / quantity per pax was very strictly enforced.
Three flights in a row now delayed with BA....why is this so difficult for BA??

Were all 3 of the flights, you mention in your last sentence, delayed due to cabin baggage?
Or were you just annoyed because the 3rd flight was delayed and therefore the reason was cabin baggage but it could have been a different reason, as there are multiple reasons why flights are delayed, therefore you would have been annoyed regardless, understandably when its your 3rd flight in a row!
Think of the passenger not in CE and at the back of the plane that may of had their luggage checked in after boarding! It could always be worse! Hopefully your next flights are smoother and trouble-free.
As a side note my tactic, I always try to bag (no pun intended) 1F or 1D, is either board 1st and people watch with delight, as I do, therefore avoiding any overhead locker problems for yourself, as these are challenging in row 1.
Boarding halfway through the entire process can be stressful for everyone involved watching and waiting for others to board and find seats etc, this could have been avoided as a CE pax, unless you were late to the gate!
Happy flying.