@ Doctor Syntax
In the case of the Swansea Lagoon, the environmental effects have been studied probably more extensively than any other proposed project, and found to be extremely benign compared to other human things. Like houses or cars, let alone power stations.
One of the followup projects planned by the Swansea folks - building on the Swansea experience - is a bigger lagoon at Bridgewater, that'll not only generate power, but also help protect against flooding. If you believe the Somerset Levels are land - and value peoples houses, businesses, farms, and some of our best cheeses - that sounds like a positive benefit.
London's postal vans want a depot somewhere out Essex way, with some local tidal capacity. It won't be as much as in the Bristol Channel, but those Thames Estuary and North Sea tidal flows are still strong enough to be clearly visible from space.
Biting the hand that feeds IT
