Surely there are a couple of very large projects that could generate by tidal power. They could have significant environmental or shipping impact. They might include barrages across the Cardiff Bay and an even bigger one from the north to south coast of the Avon valley - the river has a very high tidal surge. An even more extreme one might be to use the water passing the narrowest part of the Channel/Manche between Dover and the Pas de Calais.  

Do you have any statistics on what excess has to be built into the system to ensure constant supply when the amount available from renewables is low - if there is a long period of calm winds for example?

Like Jerome I would place nuclear fission low on a list of options and would want equal or greater investment in micro generation and renewables. There are considerable resources lying disused in the UK where historically the water courses have been shaped to provide things like mill runs so that water grain mills and later water powered industries thrived. These fell into disuse with the introduction of steam but a lot of the infrastucture is still there if hidden under overgrowth.