About 30 years ago I was involved with the last serious study of the Severn Barrage, under the direction of Tony Benn as Secretary of State for Energy and Sir Hermann Bondi, as chief scientist to the Department. Much of this study is relevant for the present day, subject to improved modelling capabilities.
I find your account
confusing. At one point an output of 8.64 MW is mentioned, presumably installed capacity for peak output on a Spring Tide, but then the article goes on to compare the project with nuclear power, for which the issue is continuous power, for base load, not intermittent energy generation - in the absence of the elusive elixir of massive, efficient power storage.
The Severn Barrage can be designed for an infinite choice of balances between energy and power, but the cost of the project (per kW or kWh) varies markedly and the overall national energy plan must determine the best mix.
The high tidal range in the Severn depends critically on magnification of the tidal wave, by resonance, affected by the length and shape of the estuary. The lower in the estuary, a barrage is placed, the greater may be the reduction in the tidal range, affecting power generation.
ALAN MUIR WOOD, firstname.lastname@example.org