Assume a manuscript written in AmE (if it matters, also assume an American MC).
It would be relatively easy to change spelling, word choice, etc. to make a BrE manuscript. However, since I write with an AmE brain, I feel that such a change would be somewhat lacking (not to mention, I might miss something).
How bad a tactic would it be to send a manuscript in AmE to a British literary agent - keeping in mind the MC is American anyway?