It is because of localisation. The BBC does not show the same content in all regions. The BBC supports 12 individual regions in the UK. News programmes being the most prevalent. This is how they realign from local content back to national content.
Is BBC news channel localised internally? I thought it was just on the mainline channels for things like the evening news.
I'm a little rusty on regions/nations stuff, but there are a lot of opt-out/opt-in points for the evening news, and something like countdown runs on network in case a region/nation fails to switch to studio.