So this is what the country’s power usage looked like over the past 7 days:

…and this is what it looked like for the past 24 hours:

…and this is what the deviations from the 50Hz median frequency looked like:

…all of which are available from the National Grid Realtime website; of the latter they write:

The normal system frequency is 50Hz. As electricity cannot be stored, the instantaneous generation must match the demand being taken from the system. If the instantaneous demand is higher than the generation, the system frequency will fall. Conversely, if the instantaneous generation is higher than the demand, the frequency will rise. System frequency will therefore vary around the 50 Hz target and National Grid has statutory obligations to maintain the frequency within +/- 0.5Hz around this level. However, National Grid normally operates within more stringent ‘operational limits’ which are set at +/- 0.2Hz.

Some digging around on Google suggests that the frequency shift is even more tightly controlled than that, and that ‘large’ (presumably > 0.2Hz) deviations are used to indicate peak/offpeak power availability to industry – but I can’t find anywhere confirming that yet.

Time to check wikipedia.