BBC 1 and BBC 2 are"reactive" channels, with lots of live content all day. The live content is never taken off-air at a fixed time... it comes off when it is finished.
Well you could have fooled me. I've often seen the weather forecast chopped with the presenter in mid flow, because they (the presenter) are overrunning by a couple of seconds.
Ok, you've got me there.
On network BBC 1 the Weather is generally taken off air by the automation... mainly because the Weather presenters are generally very reliable at following a count and stopping talking at exactly the right time.
Sometimes the nations will come off the network Weather manually, if the network director has not tidied up the off air time to a full second... or just because some announcer/directors in the nations simply prefer to come off all network Weather forecasts manually, because of that risk of the presenter running over.
Network have also, on occasion, come of Weather manually if they are aware that there is problem and the presenter is going to over or under-run.
However, Weather is the exception... not the rule. All other, unpredictable live content is taken off air manually.
And of course, not all network Weather forecasts are presented live.