:That's a very normal number, wind farms make, on average about 25% of the maximum strength, but sometimes it will blow at around 100% and sometimes it won't blow. That average is known as capacity factor, it's normally between about 25% and 35%. It's also partly to do with the design of the wind turbine. It's mostly a different thing to how much wind can be extracted from a particular region. And note that that's not an efficiency thing; wind turbines are cheap to install, they don't need all the boilers, condensers etc that a normal power station has to have, so they can produce very competitively priced electricity, below that of nuclear for example (at least for on-shore production). 

 :The UK is particularly well situated. First it's on the west coast of Europe so the wind that reaches it is unobstructed by mountains. Second the jet stream is aiming pretty much straight at it. Third it has a large coastline that can be used for off-shore wind farms, and the UK is reasonably flat, which means more places are available for on-shore wind farms, it gets less wind shadow within the country than many other places. There's probably other factors as well, but you get the idea.