The IncludesVariables.xml will show the UK Flags if the ratings are stored as 'UK:**'
You can either modify the IncludesVariables.xml or update your library using something like:
UPDATE movies SET C12 = replace(C12, 'Rated ', 'UK:' ) WHERE c12 LIKE 'Rated%';
UPDATE tvshow SET C13 = replace(C13, 'Rated ', 'UK:' ) WHERE c13 LIKE 'Rated%';
Alternatively you can use m4x1m's script to update the ratings. See this post for more info