• The differences between American and British English are rather minor. Usually you can get away with simply copying the source string (Ctrl+G in the transifex editor). However, double-check carefully before confirming (Alt+Enter).
• Use British orthography and vocabulary, obviously. For more information, see wiki:EnglishStyleGuide#Englishspelling
• Check you use proper Unicode characters; see wiki:EnglishStyleGuide#Unicode
• Replace double quotation marks “ and ” with single quotation marks ‘ and ’.