In that sort of situation I might say… ‘very sorry but I paid for that specific seat’.
Technically I might have ‘paid’ by flying a lot on BA to achieve status… so though I might not have shelled out cash specifically… but ‘pay’ I did.

In any case… life is to short to worry about childish behaviour like that person exhibited.

PS Just to add that I speak and/or understand quite a few few languages. So in many cases like that I would able to say something along the lines of ‘very nice to meet you’ or ‘hope you have a nice stay in XXX’ at the end of the flight. Though only did that once in a similar situation… the look of horror on their face was worth it!
You might want to invest in learning a few more languages so you can enjoy doing that in future.