IMDB is not a good source for series. BBC currently have them as one series: https://www.bbc.co.uk/programmes/b008vj2j/episodes/guide So I can't spit them unless someone find undeniable proof that they were first released as individual series.

Of course they are sequels and must be putted in correct order but nowhere the network names those different shows as "seasons" of one, neither lists them by giving a number of season. They started to create a series of serials, as Wikipedia claims too, they use the same character(s) but they decided in time when to produce and release each one of them, without specific periodicity. Without specific release dates, number of episodes and so on. They did three years to produce the last series of those. If you open links you gave me, you'll read about "Two-part psychological thriller", "two feature-length episodes of intelligent terror" or "in a three-part psychological thriller" and so on. It's crystal clear thay they advertise each production separately and not seasons.

You may want to check our TV guidelines: https://www.themoviedb.org/bible/tv/59f73eb49251416e71000026

You mean that "season" worldwide is calling "series" in UK? And why we should follow that term? Not any country by itself or network or anything can dictate terms in a research and recording of television/cinema history. Only facts and data should be considered to record them properly. You asked me for proof and I gave you. Networkd itself promotes different productions on their webpage, as I mentioned above. Besides that, if you are by the book, then according to this there is no "Messiah" series, but only "Messiah: The Rapture", when that "season" came as final on 2008. Same goes for "Spartacus" that should be renamed as "Spartacus: War Of The Damned", "Gotham" that should be immediately renamed as "Gotham: A Dark Knight" etc.

The original network is king! And, btw, the guidelines for seasons names are here.

The "original network" is not Academic Knowledge. So if you do not want to stay in coffee-table discussions, the structure and rules should be of academic standards and not counting what any producer or production company dictates for himself/itself.