the key issue here is the cost of deep (> 30 metres) offshore wind energy.  if 2 turbines with average output 4 MW cost 162 million (is this GBP?), installation cost per GW is about 40 billion GBP.  For comparison, nuclear power stations cost about 1.25 billion GBP per GW to build.  

Shallow offshore wind energy is much cheaper, but even for the UK realistic estimates are that the area available can't provide more than about 100 TWh/yr out of a total energy consumption of 2700 TWh/yr.