The National Grid is responsible for maintaining the electricity supply to the whole country. They generate and buy electricity from a number of suppliers, both private and public, and then distribute it via power supply companies who sell it to individuals and industry. It is a very complex issue to constantly balance the generation of power with the national demand second by second as very little power can actually be stored for future use!

This need to buy (or sell power to other countries) at short notice gives rise to the far from straight forward pricing for the cost of generating power as it has to be fair to all the suppliers so that they can all stay in business. 

There are several very good website dashboards available that provide excellent information on the performance of the grid system in real time (typically updated every 20 minutes). They show the national demand for power (in gigawatts – GW) and  how that power is actually being generated. Some add other information such as cost and carbon production. My favourite is https://grid.iamkate.com