Assume they are insecure and looking for a passive-aggressive way to cut you down to their level. The British^W English are quite adept at doing this in a way where you won't realize until the next day that they were insulting you.

Additionally, don't attempt to banter even if the Brit initiates it. Brits think that Americans cannot banter and will assume that any banter from an American is intended as a sincere insult. Don't try to explain that Americans frequently banter among each other and no offense is intended, they won't believe you.

It is better to remain cordial as they 'banter' at you, but to never reciprocate.