Two in Wales that I have visited, Dinorwig opposite snowdon and a smaller one above Blaunau Ffestiniog. Both are closed systems to avoid liquidising fish and from the looks the water is dosed with something.
Of course these were built to soak up off peak electricity from (primarily) nuclear power stations and dish it out at a much higher price at peak times, now they will make use of peaks in non scheduled generation like wind and solar, especially as we lose our last nuclear generators (and the French and Chinese f*k us over with the three they are stop start building.
Both the above use enhanced natural reservoirs, the Dinorwig one is 600 metres head so not many places available, if you go for a reduced head you need a higher mass of water per unit energy, this involves a more massive infra structure and cost tends to scale with mass.
In germany I believe they store energy as high pressure air in underground caverns.
Well I agree to a degree, many of the weirs on the Thames and it's feeders could have Archimedes screw generators like the one at Eton but I don't know their rate of Return on Investment. The EA recently rebuilt a weir near me on the river Wey, the original plan was to install a screw generator (because they are fish friendly rather than particularly efficient) and an eel and fish pass but the generator plan was scrapped and EA will not tell me why.
With generation plant you have to consider capital cost, operation and management and fuel costs. Even with zero fuel costs the other two can add up to expensive power.
You can do a similar sum with battery storage and as far as I can see the capital costs for the total amount of energy stored in the battery lifetime are still currently higher than I can buy electricity for.