There are ~30m vehicles in the UK. By the time gas turbines are decommissioned most will be replaced by EVs each of which will have a capacity of averagely 60kwh.
If all were fully charged and vehicle to grid (V2G) capable, they would provide 1800gW - enough to power the UK for for 60 hours at 30gW per hour.
I know this is glass half full stuff - but assuming just 25% of the potential would be 15 hours.
I expect gas turbines to be decommissioned as they age over the next 20-30 years, and nuclear capacity is planned to provide 24GW by 2050 (pigs might fly!!)
Statistical analysis of weather and demand can estimate the probability of a power deficit in adverse conditions. Extremes which fall outside a probable worst case will need a contingency plan to selectively and sensibly load shed.
The alternative is costly - maintaining a power supply under the most improbable circumstances. A balance needs to be struck - as is the case with gas at the moment where storage covers only ~7.5 days during winter. A failure in gas supply network would leave UK without power within 2 weeks.