I can confirm that this list exists and is frequently used

As the previous Labour Party librarian, I can confirm that this list exists and has been compiled from the electoral register. The central Party office then works with local committees and particular individuals to list the political allegiance of households and individuals. The list is indeed used for many purposes including to solicit votes prior to elections and to confirm Party loyalty for government jobs and positions. The list is for example used for soliciting votes via telephone calls on the day of the election.