No people from the UK, or New Zealand, or Australia would be "perfect" unless they're demonstratably masterful at speaking with an American accent.
We already stupidly have Xavier and Magneto inexplicably British sounding we don't need any more bastardized characters because dumb critics and movie buffs conflate sounding British with being a better or smarter sounding actor.
The only actress who get trotted out as being "perfect" for Emma who is from the afformentioned foreign countries who's shown to be at all good at sounding American (or if you want Ameri-Canadian) is Nicole Kidman. However she doesn't look particularly like Emma and she's an A + lister which rules her out in any logical persons book.
You want to cast Sienna Miller for something? Cast her for Caucasian Psylocke. You want to cast a British dude for something? Cast him for Sinister. Because unless they're master character actors like Gary Oldman or Christian Bale the odds way heavily toward a director just telling them to forget about immitating the accent the character is supposed to have ala Bryan Singer with Xavier, Magneto, Pyro, Colossus, Storm in X-2, etc.