Simplest answer, productivity.
Wind generates very little in regards to the requirements of the nation. One wind turbine is enough for a street, so you'd need one for every street in Britain to meet the minimum requirements, realising that makes you see how wind turbines are not a cost effective solution (and they kill around 300,000 birds a year). Same issue with solar power. For example, to power the UK, you'd need a solar panel the size of Texas. However you could put solar panels on every rooftop of every building in the UK, giving roofspace an additional purpose; generating electricity. But solar panels aren't cheap to make.
Hydroelectric power is much more efficient. The UK would benefit from them, but it's dependent on hydrology, uses up large land masses so more disruptive for local communities in landlocked countries and devastating to the natural wildlife in some areas due to it's size and production requirements.
Coal and oil are simple, efficient and familiar, problem is it's slowly killing us. Oil uses very little yet returns massive energy output. Generates many other spin-off industries and jobs. But it's main problem is that it's finite.
Nuclear fission power is exceptionally suited for energy production, France for example utilises roughly 60% of it's energy needs on nuclear power. It's also cheaper to generate than oil and coal. Zero emissions, making it a green energy production, of course there's the elephant in the room; the waste product. Nowadays the nuclear industry has become very efficient and reducing the amount of waste and even reusing it. Of course this makes the waste even MORE hazardous. The holy grail is nuclear fusion reactors, which cannot go into meltdown, easier to control, even more efficient and effective that fission, fuel products are limitless and virtually no waste. There's just one tiny flaw in the plan;
We don't know how to generate the temperatures high enough to make it work. Think "the Sun".