As Google is set to American English, and suggests answers with American spelling, are we missing out by only considering UK English spelling when commencing a Search Engine Optimisation / Search Engine Optimization campaign? [artBreak]

Simply put, YES!

Although it is easier when optimising / "optimizing" a website now in so far as we don't have to consider upper and lower case anymore. We still have to consider whether we should be spelling the English way, or the American way. 

Although we are in Australia, using UK English as a language, Google is defaulted to USA English, thereby assuming spelling errors when we have actually spelt the word correctly. For our case, the word "optimisation" is suggested to be "optimization". A client of iEnhance Pty Ltd are a "sterilisation" company.....Google much prefers "sterilization". Therefore, if we focus on one: eg "sterilization", we are missing out of the UK speaking Australians who would type our correct grammar in to the search bar.
Depending on budget, I personally would look at both, but at a pinch, I still go for our traditional spelling in the hope that Google may one day let us keep our own spelling, and not force the global language of American English on us.