He grew up there, at least part of the time. His mom was British.

Then why does constantly mock it?

Do you have an example of Britain being mocked? I mean, both shows consistently have British guys as main characters (Giles, Wesley, Spike), which is a big thing considering they take place in California. I can only think of situations in which characters banter with each other like they also do with the United States.

Okay fair, guess it is just banter, just feels weird that it happens a lot.