Depends what you mean.Isnt most "reserve" for electricity generation held in the form of pumped storage hydro?
That's the conventional criteria, Dinorwig pumped hydro can store around 9GWh of electricity, which is a lot, almost a third of a day's power for the UK.
We used to have 14 days worth of Gas storage. If that's now down to 30% of that it's still 4.2 days of electrical and heating power that's saved up. Our Gas plants run at around 40-45% load. That gives a lot of room to run them more of the time.