We took the 390 to the Embassy (we lived right near the route's start in Archway). BUT we were travelling at around 6.45am, with minimal traffic. Since you have a 10am interview, I would allow at least an hour, and probably more, to get there as you will be travelling down Oxford Street -- a notorious bottleneck, even in the mornings -- for much of the route, whether 390, 10 or 73. Just a tip!
If you don't mind a little bit of a longer walk (and by that I mean about an extra 10-15 minutes), your best bet is to take the Victoria line straight to Oxford Circus. Two stops, no changes, at most 10 minutes underground. If you'd like to walk down Oxford Street, the nicer shops are generally after Oxford Circus anyway (and Top Shop is right there!). If you'd like a quieter walk, you can have a jaunt through Mayfair before you get to the Embassy -- walk south on Regent Street, take a right on Brook Street or Maddox Street and you'll hit Grosvenor Square.
Just a thought!