Remove all Americanisms from the writing. This includes spelling, wording, and context. This means you should use English, English spellings, not American spellings: For example instead of ‘Color’ it should be spelt ‘Colour’. You should use English wording not American wording: for example instead of ‘locker room’ it should be ‘changing room’ or instead of ‘gas station’ it should be ‘ petrol station’. You should also use English context, for example the mention of secondary school instead of high school. These changes should not only apply to the examples above, they should apply to all possible scenarios and forms. In summary remove all Americanisms in place of Britishisms. This doesn’t mean changing how characters speak by adding abbreviations to try and approximate an accent, rather genuinely changing the way it is written to reflect British English.