[Possibly final update] I wrote a polite, but very firmly-worded letter to all the executive contacts on the Elliot Advocacy webpage. I explained why the original refund offered was wrong and cited the UK261 regulation plus the precedent of the Steef Mennens case. Then, I told AA they were in breach of their obligations according to the UK261 regulation and explained that, without a response before the 10/11, I would be forced to take actions to obtain redress.
Today I received an e-mail from a more senior CS rep, who said they'd contact the Refunds Dept to adjust the amount according to the Steef Mennens precedent. A further refund followed shortly afterwards, which, in addition to the first one, roughly corresponds to 45% of 75% (if that makes sense) of the fare + carrier surcharges paid.
All in all, my wife and I are satisfied and consider that the clear articulation of the rights and judicial precedents ensured we received what was duly owed.