The irony is at the time of me writing this; gas is down to 4.6%[^1] and renewables are a whopping 88.5% and yep - the cost is based on the 4.6% of gas.
I think gas turbines can turn off completely without issue (unlike coal) but there may also be situations where it costs more money to restart the generation process completely than to idle it at low capacity for a few hours when there is no demand for gas generation.
Now, the CMA report that's linking is talking about a world we no longer live in, in that world the UK burns coal, Russia hasn't invaded Ukraine and so on, and thus the numbers might be entirely different now, but that's the best I could find.