Anything which goes into a net dies unfortunately.
The French one has the advantage of a huge pool behind it and relatively small inlet. The severn one is going to be a collosal barrier.
There are actually plenty of places in Scotland with the same topography but unfortunatey unless you want to power sheep sheds they are hundreds of miles away from the demand.
Then there is the UK planning permission side of things not to mention that the queen owns all the sea bottom. Then building in high currents is an utter bitch as proved by the thames barrier.
Sailors are anti anything they may bang into. They know its only a matter of time before something hits it. Then they are opposed to controlled waters as much as pilots are with restricted airspace. And the big boys hate anything which slows there access to a port and you would need lock gates in the severn.