You can fly a Red Ensign anywhere on a boat but best from the "blunt end".
It is more of a national thing than a rule so as long as when you are under way it looks tidy then that is OK.
One point. It is unlucky to remove a worn Ensign until the flag has disappeard (the union Jack) Hense the term "It is good to see the tatered Ensign". It came about from the old days when boats used to go to sea for years a time.