Don't forget it's not that long ago (last season) that Adam Roynon was riding in all 3 leagues!
I think Scrammy (on a 3.5 PL ave) can ride in the NL next season, but, for instance, Lewi Kerr (just over 6 PL ave) can't.
I still think it's daft that, say Jason Doyle, can continue in the PL despite being an EL number 1, although the EL isn't as near as strong as it was a few years ago (the amount of PL riders in it proves that).