Hi, major.

When the show changed network and cast in 2017, also the series counter officially reset (by Channel 4's choice) to "Series 1". That's illustrated by Channel 4's official site for this show: https://www.channel4.com/programmes/the-great-british-bake-off

Unfortunately, this type counter start-over occurs for various well-established series (for example American Idol, in the States), and it understandably confuses people. Regardless, whatever a show's new channel decides to go with, for Series #, is what's "official", thus is "the final word", therefore what TMDb must go by. Otherwise we'd be presenting series #s that are inaccurate.

Yes, as @genplant29 said we always follow the official seasons. It's all explained in our Contribution Bible: https://www.themoviedb.org/bible/tv/59f73eb49251416e71000026