I didn't realize Jean Simmons was British; I suppose I know her best from her many American films, in which she did an excellent job of suppressing any accent. She certainly gives no hint of her east-of-the-pond origins in, say, Guys and Dolls!

(Confession: I've never sat through the Olivier Hamlet)

I've always thought that she and Audrey Hepburn shared a fairly close physical/facial resemblance, and that this would have been one major reason that the then-unknown Audrey got the Roman Holiday gig as second choice. The two were very similar (and very classy) "types," so it's no surprise that many folks would perceive their acting styles as similar.