Really the appropriate thing for BA to do would have been to apologise profusely for undercatering the flight, explaining that you should have had catering from London (sounds like something along this lines was said, as OP says the interaction at the gate was “quite lovely” Full catering for an inaugural flight is hardly a big ask, as there is a good chance it will end up full!

But then they should have proactively offered that OP purchase suitable food for a 4 hour flight if available in the airport, keep receipts, and claim back as they could note on the booking what had happened. It would then take awhile, but I’d expect reimbursement as well as an Avios gesture with an apology email. That’s what the BA I used to know would have done…

Sounds here like the interaction was a smiley and nice crew saying, “Well, we were not able to give you a part of the product you paid for… Do have a pleasant flight.”