Those numbers seem off. Lets assume you not cutting it off right out at the sea, but a bit closer in, for the simple reason that a dam 50km in length is well probably not going to work in the deeper areas, ecologically or economically. Closer however we can assume a triangle shape and lets be optimistic and assume 20km wide and somehow the greenies sign off on that (seriously ecologically this is a really hard sell). That 20km wide 20km long with a 5m head. This is very unrealistic, we are assuming we can empty and fill the lagoon instantly at peak head: 20kmx20km/2*5=1Billion m3. Potential energy with a 5m drop, rise x2 drop x2 is 1e11J. Or 27MWh per day. This is no where near 25% UKs power needs. Wiki claims average consumption of 840GWh per day.