I can only speak for Wales but...
Magners League: Wales do not have the quality outside of the Regions that could possibly enable a system where teams are relegated and promoted. It's just not feasible, strength in depth and quality are not there.
Guinness Premiership, however seems to be in a much healthier position. I mean, it's simply a league made for 13 teams, but there's only room for 12. One team gets relegated from the premiership and it bounces back the very next season, same old story.[/b]