Final update on this from me,
I just randomly logged into another BA Executive Club account under my name, which I've never really used before. To my surprise, I noticed that the original 5000 Avios were deposited into that account. It's quite perplexing because I was logged into my main account when I submitted the complaint, and I definitely provided that EC number.
Consequently, due to BA's error of crediting the points to the wrong account, I have now received a total of 15,000 Avios for the inconvenience of missing a kosher meal on the RTM-LCY flight.