I've never seen the fares differ, at all. So my guess is that the "US" one priced out as a one-way while the "UK" priced as part of a round trip. One way TATL fares in economy are often very high, while tacking on a return can often bring the total price below the one-way was before.
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Thread title has been updated. $1700 is expensive for a one-way coach fare, but definitely not extortionate. 
~Moderator