Pump-storage hydro is for load balancing as it can’t store anything on a grid scale and is very inefficient. Sadly, it could never replace gas — the U.K. alone would need thousands.
It's grid-scale load balancing, not grid-scale storage. Grid-scale storage is hundreds and thousands of GWh. The UK needs 30-50 TWh to be secure without fossil fuels.
https://en.wikipedia.org/wiki/List_of_pumped-storage_hydroel...