In the worst case situation, HRH passes away on a Sunday morning during a Roger Johnson and Steph McGovern cover shift. Would they really leave it to those two?
Well, Roger Johnson has done hundreds of hours of live television, so he's not like someone on work experience. Steph hasn't done so much general presenting but she's clearly highly intelligent - of course she was a producer for many years before becoming a presenter - and they trust her enough to cover serious news. It's no different to if the news breaks at any other time of day and they have a less experienced presenter on.
Besides, no presenter currently working in news today on any channel has experience of reporting the death of the Monarch.
If it was a situation like Diana where the initial reports are of an accident etc Breakfast might do the early reports then hand to London once it is confirmed as an obit. Not because they're not capable of covering it, but because the team in London are the ones who regularly rehearse it.
I wonder what would happen if the same circumstances in which a royal met their death also claimed the lives of hundreds of members of the public? For example, if an event which a senior royal was attending was targetted by terrorists, would the broadcasters stick with the "drop everything and go into obit mode" approach or continue to cover it as rolling news? I could imagine families of the other victims - and potential victims - being unimpressed with the coverage turning into sycophantic fawning over the dead royal with the rising death toll being reduced to a mere footnote in the coverage.
Obviously it goes without saying that I hope we never have any such scenario and never find out how they really do it.
Write that down in your copybook now.