The feature-length special appears to be currently entered as a separate "movie", at https://www.themoviedb.org/movie/664806-red-dwarf-the-promised-land

If UK live-action series follow the same rules as anime and tokusatsu series, then because this "movie" was first widely shown on broadcast TV, not in cinemas, and it continues the story of a TV series, it should be stored as a special of the original TV series, not as a separate title. But I'm not sure that they do follow the same rules. We'll have to see what the admins say.

You can find a previous discussion about this issue here. To answer Seragon's question, The Promised Land is not series 13.

Going on what was pointed out in that earlier thread, I refute my previous post here and agree that it's correct that it should be stored as a separate TV movie (as it has a separate mini-site on the original broadcaster's website), at least for now. If there is another season of the series that is set after it, that might make things complicated, but until that happens this seems to be right thing to do.

As it is the next main part of Red Dwarf, not a spin-off side story, it should not be duplicated in the series' specials (as it currently is), and it should not be split up into two parts, because that is not how it was broadcast on TV (and not how it is intended to be watched), only how it was added to the broadcaster's on-demand service for technical reasons.