I'm just cynically generalising - the rest of world compared to the UK has typically less stringent planning requirements, and more mountainous terrain. To store energy in other countries (except perhaps Holland..!) you build a hydro dam, and pump water into those dammed lakes using excess electricity to store it up for later release.
We have mountains (Wales, Lake District, Scotland), but the likelihood of creating lakes for hydro use is close to zero. And you would need many, many, flywheels to substitute for one lake.
