[Sebastian wouldn't answer, since others are already taking care of the explanations and there's no point in being redundant... but that's the first British accent he's heard here (aside from his own and his young master's) since... oh, when miss Moriarty was still around, probably. Greeting your compatriots - so to speak - should be a thing, he supposes, when your master serves the Queen.]
Unless I am mistaken it seems others have already explained the situation, but should you have additional questions I would gladly assist, sir.