Haven't been to Dinorwig, but I've been to the one at Cruachan. Interesting place. Wikipedia says the only other one in Scotland presently is Foyers. I thought Glendoe was pumped as well but apparently it's conventional hydro, is that the one you're referring to @TomH?And the rest. I think UK has about seven if these with a new one under construction in Wales somewhere.
Dinorwig is the biggest I believe. Google Pumped Storage Hydro for all the details.
We need more but suitable sites are quite rare. Also occasionally fouled up by our free market system and it's ill-conceived rules. These schemes actually generate little new electricity so thet don't attract green brownie points for the operator. So the latest new scheme in Scotland was a straight hydro (green) occupying a perfect Pumped Hydro site. Nice high drop into Loch Ness.
Planning - what planning?
Yes Glendoe was the opportunity missed.