Two points you are missing is that
The weather isn't a local story it is a national story. Yes it is only occurring in certain regions, but if that was the case most national stories would be local stories and never appear in the national news.
Secondly, nobody is complaining that Mandela is the top story, but why can't they break off to feature the other news of the day, especially when the bulletin has been extended.
Point taken on the weather, but I think I should point out there that the weather was not the only big story today that got dropped from the BBC News at Ten, so did the Channellor's Autumn Statement, and that is at least as important. In fact, there was no coverage of any other story on the BBC News at Ten, though the Autumn Statement did get reported on Spotlight in my area.
On the second point, you are missing one key point here. The news broke less than half an hour before the Ten went to air. When Margaret Thatcher died, the One went into breaking news mode, when Nigel Lawson resigned as Chancellor back in 1989, the Six went into Breaking News mode. It's happened time and again throughout the history of BBC News, stories breaking just before the next scheduled BBC1 bulletin, that bulletin goes into breaking news mode, so it's not unexpected that they dropped everything else for Mandela.