Could you not just keep gas as a backup? I don’t see why you need both coal and gas?

I suppose gas and coal are sourced from different places and that may give some resilience to the system. They also serve very different functions in the grid. Coal is for baseline and gas is for peak.

Coal is very much used only for peak in the UK in recent years. The remaining plants (in fact, there’s only one still in service now) are kept on standby during winter and activated only when demand is forecast to exceed supply (plus a safety margin).

Gas is now more of a year-round flexible baseload, taking up the slack whenever demand is high and/or renewables production is low.

Nuclear is now the only true baseload in the UK - always generating more-or-less the same output except when they have to be shut down for maintenance, refuelling, or decommissioning!

These days, it's nearer the other way around in the UK - gas is closer to baseload (sort of) and coal has a profile closer to a peaker (especially when turning up/down rather than on/off).

There was an issue with gas in the 2022/23 winter as the Russia-Ukraine war affected supplies and caused prices to skyrocket. More coal than usual was burnt because of this. But it's a moot point now: coal-fired plants have continued to close in line with UK government policy, and the last remaining one is scheduled to close in September.