I’m not entirely sure the London presenter would be straight out of the door, unless her car is on a meter outside.
If she really wasn’t able to continue, there would always be someone who could read the autocue in a half decent manner for a bit, even if not another presenter, there would be on screen reporters like Joel Hills about. The London late presenter might be there, Tom Bradby might probably be there. Peston might be kicking about, although that would be an interesting prospect.
I'd have thought Tom Bradby would be there, if no one else, considering that the anchors typically arrive a good few hours before their bulletins go to air. Slightly off topic, but I watched a VRT bulletin a couple of months ago where the presenter, much like Mary Nightingale the other night, had a sore throat, and so the sport presenter took over midway through.