[broadgage]

To add to the points already made regarding frequency control, any sudden and severe drop in frequency also triggers the automatic starting of gas turbine and diesel generating plant.
If the frequency remains low, then areas are automatically blacked out by frequency relays in substations.
Such drastic measures are only taken if the grid frequency drops to about 48.8 cycles, well below the normal lower limit of 49.5 cycles.

IIRC this last happened some years ago and resulted in large scale but brief power cuts. There was nothing exceptional about demand at the time, the problem was caused by the almost simultaneous tripping of two large generating units.
The frequency remained below the legal limit of 49.5 cycles for about 10 minutes.
The national grid are required to have a margin to cover for the largest reasonably foreseeable failure, but if two failures occur at almost the same time, not much can be done.