Sizewell B nuclear power station is operated by British Energy and is the UK's only large pressurised water reactor. In the first ten years of it's operation it produced about 3% of the UK's electricity. British Energy claim the power station has reduced the amount of greenhouses gases that would otherwised have been released by fossil fuel generation by around 60 million tonnes.
However they have conveniently accounted for only a tiny part of entire production cycle. A large amount of energy (and therefore greenhouse gas emissions) are involved in the mining of uranium ore, it's refining and enrichment and the construction and eventual decommissioning of the nuclear power stations themselves also needs to be factored into the equation.
Last year, during the climate camp near Drax in Yorkshire, protesters blockaded the Hartlepool nuclear power station.