To the OP,
I was on the same delayed flight as you and had a similar set of knock back responses from BA. My last communication to them was about a month ago, asking them to confirm that their final word was ‘no’ and if so then I was willing to go to to CEDR - their reply was the last standard one you got (“we’re continuing to investigate”).
Anyway, the good news is that without me doing anything else I got this reply yesterday;
I felt the need to write back to you, as we've completed our investigation about your delayed flight. I'm sorry for the delay in replying to you.
I’m pleased to advise you’re entitled to compensation for the delay to your flight BA2659 on 17 December 2018.
The distance of your disrupted journey was 1,500km or less and this has been calculated in accordance with EU legislation. This means you’re each entitled to €250.00 in compensation. The total amount of compensation due to you is £430.94, which is equivalent to €500.00, as there are two passengers included in your claim.
I don’t have any experience in these matters but my guess is that their grounds for refusal has been tested via somebody taking the same case further and they have lost, so they will lose all other cases for this flight.
Anyway, hope this is a bit of positive news.