Worth knowing that the way BA appear to process claims is that they look at the reason code on a notification, and don't initially investigate beyond that. If the claim is pre-assessed to be EC261 friendly, the ops team will set the last letter to "Y" for "yes" on the notification. For operational delays, you could have either OPER or OPEY. The latter would be agreed immediately, the former is really indeterminate.

However the first level of sifting assumes OPER as "N". So the process is they generate a form letter, as the reason code was OPER they say "Operational Reasons", and that's it. Operational delays are very much in the scope of EC261 so you have to push down a level and explain your reasoning to BA. It's quite usual to have a couple of rounds of ping pong before a resolution one way or another.