Great question-- the actor himself (Michael-James Wixted) is from Santa Monica, CA (obviously American); the character probably learned how to talk British from exposure to a lot of British media, or more likely, the people behind the show taught the actor how to talk British for the purposes of...
I see that this ABC dramedy was made at CBS Studio Center in Studio City-- IIRC, a later ABC comedy was made at another CBS facility (Three's Company, which, IIRC, was made at Television City for at least some of its 1977-84 run).
That's personally why (as I've said many a time) Diff'rent Strokes and The Facts of Life have always been ineligible for my DVD collection-- that (IIRC) they didn't present issues in an entertaining, funny manner, but rather in such a manner as to force you to believe in/agree with the issues...