As I said in my post with the link to Debretts titled 'Foreign Titles' in this thread. I have bolded the important part. Since the Royal Warrant of 1932 the right to use a foreign title by British Subjects is not allowed. The King and Queen of the Belgians are not British Subjects, nor are any members of the ruling European Royal Houses, and therefore are not effected by the Royal Warrant of 1932.