This is correct. And the costs are built into the contract. Wind and hydro dams are very easy to control. Gas is quite easy to control. Coal costs more to start

This is correct. And the costs are built into the contract. Wind and hydro dams are very easy to control. Gas is quite easy to control. Coal costs more to start and stop, but nuclear nearly changes in output over the course of a week. Nuclear is tuned to output the base load, the level below which demand never drops. Then the other sources are layered on top. For the situations where all power sources are at max output and there is still not enough power, then the UK will pull power from France. This typically happens when East Enders (TV Soap Opera) finished and millions of people turn on the kettle (high powered water heaters of 2 or 3 kW).