It's not like the price is set for the whole year, though. The price is set each half-hour, so it does matter what percentage of the time the gas peaker plants are necessary to supply the grid. This makes the effect of renewables on prices quite nonlinear: if they can never supply 100% of the grid, then they have zero effect on the average wholesale price. But going from 0% to 99% is a large part of the hurdle, then the transition from 99% to 105% will have a very large effect on the pricing (of course, given the variable nature of renewables, this will get blurred a bit more: currently the UK grid is entirely renewables about 1% of the time. But doubling the renewable capacity will raise that percentage to a lot more than 2%).