Sorry, “thinko” there. I meant to refer to the proposed Swansea Bay tidal lagoon, which is promoted as a “pathfinder” project for other, larger lagoons.
The Severn barrage is, of course, much larger in scope and there isn’t any near-term proposals to develop it.
The Severn barage will be more, but it is a “first of type” demonstrator design and it is hoped that costs would be lower for future, larger projects.