The problem isn't so much how much coast there is but that wind power generates from -5%-100% of its capicity (yes it can go negative) at various times. With an electric grid where supply and demand have to equal each other there are 3 options to deal with the variability,
1) have natural gas power plants on stand by ready to ramp up and down or be willing to waste electricity production from coal/oil/nuclear power plants both of which are expensive and sort of beside the point.
2) hook into to a much much larger grid where your large fluctuations are tiny and easy to handle. This is what Denmark does since it's neighbors are almost 20 times bigger than it and the Swedes/Norwegians use hydro power that is much cheaper to waste electricity for regulating wind power fluctuations.
3) just let the lights go out when wind power drops.
Option 2 is not availible to Britain. Option 1 would be very expensive and would require the very pollution wind is supposed to eliminate (except for nuclear power but that scares too many people). So basically the wind power fetish of the British govt will leave Britain either paying much more for the same power and pollution but with huge wind turbines dotting the landscape or paying a little more to have power that can go off whenever the wind dies down.
Also wind power is both much more expensive than proponents claim (seems they ignore maintanence and extra power plant expenses) and more dangerous. There have been several cases of fatigued turbine blades or towers breaking off nd landing in roads/next to houses in Germany for example.
Wave sounds promising since it doesn't have the wide variability of wind but it just doesn't seem that except for a handful of cases (small isolated islands or coastal villages) it can produce electricity at economical rates.