Yes, in that internal in this case refers to members of the party and not elected officials of the party. It generally seems to come out as a choice filtered by whose local and who has come up through the lower ranks as councillors and other local officials and gained popularity and respect that way. I should add the caveat there that I'm not actually a member of any party, so don't know the actual inner workings, just what I've been told by friends who are. I think that for the Tories, for instance, candidates are put forward or put themselves forward, the central party filters that down to a shortlist and then returns that to the constituency party for them to decide vote on.