Guest here again.
Some clarification is needed.
When the voting form is sent out, there is also a list of CDs that have been released during the qualifying period. This time, there were 135 CDs on the list. Not all of these CDs have been played on Mike Harding's show.
It is made clear that choices (for best album) need not be restricted to this list.
There is no other list of artists.

Regarding the same names being nominated each year ... assuming that is true (I've not done an analysis) ... it's not surprising. This isn't the Oscars - new films each year - or the Brit Awards (pop music subject to fashion). It's folk music ... artists tend to stick around for more than a year or two.