Because there is no longer any such thing as the market rate, well, at least not a single one.
In the dim and distant past National Grid set a single price for electricity that varied according to demand throughout the day (which, incidentally is where things like Economy 7 & 10 came from). When demand was high, the electricity was more valuable and vice versa.
Then the Labour government (I think) decided that the electricity companies should move to a 'free market' system based on trading 'blocks' of electricity. So, instead of generating electricity and getting paid a demand based rate the generators had to enter into contracts with consumers of electricity (themselves included) to supply set amounts of electricity and set prices. So instead of a system based on the actual need for electricity we have a delightful system in which the unit price of electricity varies from supplier to supplier based on when it was bought and whether or not the contracted supplier can produce it at the required time (or has to buy it in a hurry from someone else with excess capacity).
Every electricity company (generators and consumers) have energy trading departments to try and juggle to fiendishly complex system and try to ensure that the lights all stay on (although the mechanics of that job actually falls to National Grid company).
The new energy trading system is one reason that the UK no longer owns most of its nuclear industry (it's French).
A generator with a diverse portfolio of supply has to balance which of those power plants are working (based on running cost and ease of start-up/shut down for example) against the energy it is contracted to supply. If that energy can't get to where it needs to go then I would assume that the generator and their customer have to be compensated. Isn't speculation and contract driven markets for essential services wonderful.
