There is a table of electricity blackout times which is contained in the ESEC (Electricity Supply Emergency Code). These have existed for some time and seem to have last been updated in 2019. Start by having a look at the ESEC and scroll down to Annex 1 which is on page 17 of that code.
There is a variable rota disconnection plan. In the example shown here you can see that there are 8 time periods of 3hrs length which begin at half past midnight. The levels of disconnection range from 1 (not many blackouts) to 18 (electricity is hardly ever on).
For example, the screenshot below shows level 1. You can see that each area A, B, C etc has it's own designated switch off times. Your area should be shown somewhere on your bill , often a capital letter in a square box.
There are exceptions to the blackouts because some areas and buildings are classed as "protected".
The lights last went out in the 1970's with reports in various papers about what life was like in those blackouts.
With thanks to the BlackBeltBarrister for drawing attention to the subject.