Originally Posted by KARFA

So I don't wish to disagree with anything you said and noted Sigwx with regards the underlying reason, but from a purely legal point of view looking at EC261 there is an argument that BA didn't do all it could since clearly when operating from the mothership of LHR they could have used other crews and aircraft to operate that GLA rotation. I know from a practical point of view these spare crew and aircraft don't really exist to the extent they would be deployed in the OP's example - but that is a choice of BA to operate in that way and this could have been avoidable. So basically whilst the ATC restrictions are fairly clearly "extraordinary circumstances" not really within BA's control, could the knock on delay to the GLA flight have been avoided and were there reasonable measures that might have been taken to do so.

If the OP's colleagues was minded to put some effort in then it could be worth a claim. BA would of course reject it, and they would have to go to CEDR/MCOL. It is possible that a well worded argument could persuade the adjudicator to find in their favour and order BA to pay up. I would say less than 50% chance of success, but certainly not zero.