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I don't know Gary. As you say, tidal is guaranteed, and has tremendous power especially in narrow channels such as the Dover Strait. However, when the tide turns, as it does twice a day, there is a slow down, stop and return, so its not a 24 hour output. Maybe that's it. Or maybe it is something to do with shipping vessels. Its unlikely to be concerns over marine wildlife as our leaders do not generally think along those lines. I imagine that the investment would be huge as it is new technology, so Government backing would be essential to this.
In addition, biomass generators could be built. There is a never ending stream of poo going into our sewers every day, which can generate gas for the national grid. Maybe George Osbourne will live up to his election promises tomorrow in his spending review and divert money away from the dirty technology into new green jobs. Somehow, though, I don't think I'll be holding my breath on that one. Too many back-handers in politics, and too much influence from the oil industry.