Hi,
In the SDC plan, power would almost certainly be landed on both sides of the Severn, ie in Wales near Cardiff and in England near Hinkley Point, where there is already good grid connectivity.
The (intermittent) generated energy really would have to be distributed across the whole of the UK since Wales alone probably doesn't have enough generation that it could turn off to accommodate the great wash of electricity as the tide moves through the barrage!
Rgds
Damon