Also may be worth considering Covent Garden which is between Waterloo and Oxford Circus. Agree with King's Road idea too. Doesn't have the big department stores obviously, but does have the same hip chain stores and won't be as crowded.
As far as someplace to eat....I've been told that there's a Carluccio's cafe somewhere near Oxford Circus. Don't know exactly where, but would be worth looking it up.
Also just south of Oxford Circus (Regent Street, west side) is the new Apple Computer store...and it is a cool place to pop into.