Well, see, they did. The one that springs to mind first is Edward I, known as Edward Longshanks due to his long legs. There was also Prince Edward, who was known as "The Black Prince", Henry "The Young King", Richard I "The Lionheart", and a couple others. If you count Oliver Cromwell, you get "Old Ironsides".

Richard the Lionheart is the obvious exception here but you'll also come across William Rufus (William II), John Lackland (John I) and Henry Bolingbroke (Henry IV) all fairly regularly if you read a lot of British history.

As for the rest of Britain, most of the Scottish kings have epithets - though Scotland was of course not conquered by the Normans much of the Scottish nobility and royalty was of Norman origin. This trend does seem to end with the House of Stuart, so you do raise a valid point.

There was also a Llewelyn the Great ruling over most of Wales in the thirteenth century, who was married to John Lackland's daughter Joan.