This is all nonsense really. Perkins does lots of work on sky - ever watch the thronecast after game of thrones?
The production company hired some "talent" and had a formula that produced a hit - it came up for renewal and they now have a hefty contract to air it via another channel.
Sue, Mel and Mary - aren't staying with the BBC, they are saying they no longer wish to be part of a show made by a separate company that going to air the show through another free to air channel. At no point where they ever employed by the BBC. They may have resigned from the show in protest - but that is all.
The contract will indubitably have clauses about schedules and named talent. The salient lesson though is however that presenters careers can be short and viewers fickle - walking away from a show on a point of principle makes it very difficult for your agent to find you work with other production companies. Since most of the BBC entertainment output is generated by 3rd party production companies nowadays it could be a pyrrhic victory.
As for the GBBO - the producers pick themselves up and sign up any number of good presenters touting for work. Lord knows there are enough food shows around.
It would seem that most of the people here don't get how television works nowadays.
Any way I can't stand the GBBO - with all its false tension and crazy closeups - its a show about cooking for love of all that is holy! Most of it not very good. Bah humbug. I wouldn't eat a cake made by Mary Berry it would have nail varnish chips in it. Bleuch