Doesn't work as well in practice... Look at California... Similar size and population as the UK, but because most of the demand for water is Los Angeles, there's often a few months of the year where the resservoirs, sorry, gravity batteries can't spin a turbine, let alone supply the first farms down stream.
The trouble is, every new housing estate that spring up increases demand on local water tables. There is only a finite amount of water in the system, whether it is in pipes, puddles, the ground or the sky.
Which means moving water away from the reservoirs to new ones in other areas. Any power they generate on each through flow only serves to pump it elsewhere. Once you start using it for irrigation and moreso industry and domestic purposes, there's less to run the turbines. Ergo less electric.
Although, in theory you can run a closed system indefinitely.