Trust dooble to have an intelligent post.
Well, it's only best if the story/characters are credible.
I don't know about the Americans/Russians or the German, but atleast I can give info on the British position.
It's a much better movie if the characters are realistic.
And having an imaginary position in a real-world police force doesn't do the main character justice.
Might as well make the character correct. It only takes a couple minutes of research if you're unsure.
Just a question to OP.
In looking for a British actor for your main character, what sort of accent do you personally have in mind?
It's just, there are multiple London accents. And you may or may not be thinking of a roosterney accent, which is stereotypical of Londoners, but it's not a perfect portrayal, and it is also likely to be atleast somewhat annoying overtime or difficult to understand.
Bear in mind that just because the character works in London, it does not mean he has to be from London.
All trains lead to London, and the character can be from practically anywhere in England.
Commutes to London (from nearby locations) would work (particularly if the Police Officer is higher ranking, and therefore not a detective or on the beat), but so would accomodation within or close to London.
So, while a London accent may do you well, might I suggest that it would be perfectly credible for you to accept any other voice-actor, who can use a British-based accent with high quality.
If you recieve a lower-quality London accent, it's probably better to accept a higher quality non-London accent over it, as it will sound better and still be perfectly legitimate for the role.
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