And electricity is less than 20% of total delivered energy in the UK

I just made some rough approximations to illustrate the difficulty. A project of this scale to deliver 10% is bad enough. I did not take into account that tide is cyclic and therefore the installation would have an RMS of somewhat less than installed capacity. Forgive me for that if you will. The figure of 5% you quote makes things that much worse.

The second point you make is another thing that worries me. To generate 20% of our electricty from renewables by 2020 is to generate 5% of our total energy requirements by 2020. Oh dear!