Or like me, you could be a UKC working for the UK arm of a US company and having to constantly spell things in the American way and changing your spell checker from UK English to US English whenever you do an official document.
It's swings and blooming roundabouts (or crossroads
) , most people have enough intelligence to garner when they need to alter their spelling and when they don't need to worry about it, Springhaze's official document rule would work well in nearly all cases, I would say with a resume you should change your spelling, not because you have to, but because it shows some understanding of British Culture, a willingness to learn and awareness of your surroundings, but on the other hand I'd never even consider blackballing a candidate for using US spellings.
We're a mixed bag of USC and UKC at my workplace and there's never been an issue, aside from the need for consistency, in any document, pick a side and stay on it!