The list of power stations and their capacitiy totals up to about 50 000 MW - enough for 50 million electric heaters. I believe we import a bit of electricity from France (see National Grid's site for details), but roughly I should think this capacity should about add up to the electricity we use.
At present, it seems the total is made up about equally of Coal, Gas and Nuclear, with a bit of oil and a bit of Hydro. The wind generation so far is at an experimental level, and the importance of Hydro-electricity is not so much the quantity as the ease with which it can be turned on and off.
The newest power stations are either Advanced Gas-Cooled Nuclear stations, or Combined Cycle Gas Turbine gas-fired stations. The older Magnox Nuclear reactors have small output compared with AGR, Coal and Gas power stations.