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We forget how much of the office traffic, done in wee early, pre-coffee hours, is driven by custom. Brits do not share our customs before their first couple of cups of caffeine (we call it "JOE").
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For American's its simple. We stumble to the right side of the hall, nod quietly (because no one wants to talk shop before 9AM - and small talk with Kelly from Accounting is painful all the time) and go about our bee'swax.
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However, Brits walk on the wrong side of the hall, eye contact makes them flustered and the mumble something (possible "moring guv'r" - I can't tell what they are saying) and then step side to side trying to get out of your way. Well, you think, they spoke I have to answer back, but I don't know what the hell they said, and so goes the "Excuse me?".
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"What?" comes their ignominious reply. You see, they didn't really say anything. The morning custom in England is to mumble - no real words are necessary, and avoid eye-contact. The exact opposite of us. And they stand on the wrong side of the hallway.
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There are many bad ways to end this exchange. For instance, trying to carry on a conversation. Or... screaming for them to get the hell out of the way. I have found the politest thing to do is to hold up the empty coffee cup with a pleading look in your eyes. They normally then throw themselves against the wall (on into a doorway if it is around) so you can pass. Apparently pre-coffee Americans have an ugly reputation.