The spelling of "ize" words turns out to be an important plot point and it seems that Mayo regards the spelling of words like "realize" with a "z" as being somehow inferior. As a linguistic pedant (he makes a good deal of fuss about words and their use throughout the episode), he would be aware that although "realise" is the more common modern UK spelling, genuine British pedants favour "realize" and don't dismiss it as a vulgar Americanization, but view it instead as the "proper" way to spell such words.