As Electricity cannot be stored all the power producers offer their product for sale to the National Grid, they work out what the demand is and buy accordingly. The traders for the power companies are trying to work out the best price that NG will accept, there are some riders in all this I think it is Nuclear First then renewables then others. As a power producer you have your price at which you make a profit and work the market the best you can, taking into account the weather, the state of other power producers plant (boiler tube leaks, turbine problems etc).If it is to windy wind turbines cannot run equally no wind. There is also frequency control and peak loading (television programmes finishing usually) As a trader if you can get it right there are riches for your company if you get it wrong your company comes off the bars. In the middle of all this there are contracts to big users who theoretically get cheap electric. It is a very very complex system.
Keith