So who draws up the shortlist? And who decides what seats have to use an all-women shortlist? And who draws up an open list?
You apply to be a candidate, the local party then chooses the candidates it'll consider and then the membership (and affliated union block votes) pick a candidate from the shortlist.
It's a bit more complicated than that (internal Party stuff always is...) but that's a good summary.
The national Party can block candidates IIRC (and pressure is sometimes put on CLP's to keep nutters off the shortlists) and after a certain point (a few weeks before the election IIRC) they can impose candidates instead (which is why there's ususally a flurry of late retirements, accopanied by a seat in the Lords usually).