Yep in the fifth season, it moved from the BBC (who had given PBS distribution rights in the US - PBS is the US equivalent of the BBC, apparently) to ITV (Channel 4 in the UK). This resulted in the mass exodus of Sue, Mel and Mary Berry who were loyal to the BBC, and didn't like the changes in the format due to the change in studios. The BBC was more educational and less focused on the contest.
Paul Hollywood stayed - because hello $$. And Prue, Noel, and another host (whose name I forget) joined. The second host lasted two-three seasons, before another one joined (who irritated me) for about two-three seasons, and finally Allison in S11.

I like the first four-five seasons the best. It's still good, just not as good. And the educational bits are still there - just not to the same degree.