I once had a very similar encounter 2 years ago with a BA check in agent at LHR T4. I complained through the Exec Club, and my complaint was acknowledged the next day. 4 weeks then passed without a peep so I thought I'd ask how it was going, and was told that they couldn't discuss it with me as it was an "internal matter". (Surely if it involves a passenger, then it's not entirely an internal matter...??) Anyway, after 6 weeks BA eventually apologised only after I pointed out that I hadn't yet received one. Good luck with your complaint!