That's "Americanize"

One thing I'd suggest, other than reiterating that, in my opinion, you should write what's comfortable for you and maybe edit later if you have to:

I don't know the nationalities of the contest round judges, but if you do write in Brit (or Ozzie or Kiwi or even Canadian), you might want to warn the judges with your contest round entries that some of the spellings will be different from American English (or different to, or whatever). While most people know that words like "humour" have legitimately different spellings in different countries, some other constructions may not be as obvious, like the one I "corrected" above. Just a suggestion.