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Thanks for the comment Laura - in reply to your questions:

1) Water currents being denser than air (but slower than wind), the energy is about 4 times as dense as for wind, meaning that tidal turbines have about a quarter of the footprint area. Neither wind nor tide are constant, but tides have the huge advantage that they are predictable - so you can plan your power generation with absolute certainty - something you can't do with wind power.

2) There are some very energetic channels around the UK coast - for instance in Scotland and Northern Ireland, and especially between N Scotland and Orkney. But also near France around the Channel Islands, between Italy and Sicily, around some of the Norwegian fjords and really anywhere where there are islands and constricted tidal channels.

3) The real obstacles are that the technology doesn't exist yet, and it will be expensive to develop and prove this at full sea-going scale. It is rather high risk for banks and venture capitalists, and really only Government can prime the pump financially.

4) The model in the video was 2.5m high - 1/23rd scale.