I think the reason that Obama has so much UK support is not that he's actually that great but that he's more like a UK citizen than the opposing candidates and we are therefore more comfortable with that. It really grates on the English by and large (and the Irish) all this holier than thou, 'in God we trust', home baking, hockey momming, gun toting, good ol fashioned, God fearing, pulpit bashing, right wing, anti rights, anti abortion, anti choice, commie hating, eco flattening, middle class white guyness whose family is in banking/law/the senate already/oil/big business/other shady and dubious dealings ( delete as applicable) and who appear, smug and perfectly coiffed, smilingly ready to trample on the poor, badly educated or otherwise downtrodden, telling them how America is the land of dreams and it's their own flaming fault if they live on the street in a box etc and if you happen to get sick well tough- die then.
None of that is natural to Europeans- we generally have a more tolerant, socialist outlook on things and the British in particularly love and are (rightly) proud of their NHS. Obama has made a leap of faith and is trying to improve healthcare particularly for the poor and disadvantaged and naturally we applaud that and find it hard to conceive how anyone could stand against it.
Obama is not great, but he's a million percent better than any Republican candidate imho.
Don't take this personally I actually quite like most Americans ;-)