About 2 years ago I signed up with Juice. Juice is an electricity tarrif from npower. It costs their same as their standard tarrif but is produced from 100% renewable sources (mostly offshore wind farms like North Hoyle). It is also run in partnership with Greenpeace who ensure that the 100% renewable agreement is adhered to. They also give £10 a year for every Juice customer to renewable energy research projects.
I personally feel that the best future for electricity generation is decentralised generation by homes, offices, etc across the country. However it will take a while to happen and for people to save up for roof mounted solar panels and wind turbines. In the meanwhile signing up to one of the many green electricity providers is a good alternative.