Robert_Charlton - 7:39 pm on Apr 18, 2012 (gmt 0)
Regarding the observed rankings, several thoughts, all conjecture, come to mind...
The algo is most often keyword specific. If your backlinks for the US spelling were discounted for whatever reasons, that could explain the US spelling rankings drop. Since you were trying to rank for the US spelling, it's possible that your backlinks are now possibly seen as manipulative... with the US-spelled links too coordinated historically, eg, or too many exact matches.
On Google.com, the American spelling is likely to be more competitive than the UK spelling is. A combination of onpage synonym matching (ie, between the US and the UK spellings) along with natural backlinks to the UK spelling might be enough to rank you highly for the UK spelling.
Google foreign tlds are also often less competitive than google.com, but perhaps less so for the US spelling than for the UK spelling. On the foreign tlds, where your US spelling is doing better, geo-location of inbound link sources, etc, might also come into play.
I also doubt that there's a consistent ordering that you can make of how algo factors are weighted for a specific site/query combination from one google.tld to another, particularly since so many other variables are in play. You'd have to do a systematic examination of trade-offs for each tld to even make a reasonable guesstimate of what's going on.