National Grid do publish historic frequency data, so it will be interesting to look at this. unfortunately they publish it some months in arrears:
Losing all of Little Barford is a surprise, since I assume it has duplicate step up transformers to 400 kV, so it might have been a 400 kV bus fault.
The subsequent loss of Hornsea is interesting, I assume that was because of ROCOF or Vector shift mains failure protection.
Vector Shift is being phased out in the UK (not a joke) because of perceived mal-operations (at least in the UK):
"On several occasions, it has been suspected that a transmission system fault that did not result in islanding resulted in the inadvertent tripping of embedded generation plants by their LoM (Loss of Mains) protection. A definite event was recorded on 22 May 2016 following a single transmission circuit fault. Further investigation of this event showed that a significant number of embedded generation plants had tripped as a result of the operation of VS protection. This event resulted in a loss of infeed and a frequency excursion that was bigger than that which was anticipated."
New protection settings are being used and there is a lot of work making this retrospective.
As Scotty says, wind generation. in particular, essentially has no inertia (because they are connected via inverters) so the system inertia is falling as the historic coal stations are being closed. This can result in potentially larger frquency excursions as may have been the case of yesterday’s event.
Well at least the transmission system load shedding worked, otherwise they could have lost the whole GB synchronous area!