In the UK we have the national grid that supplies all connected homes in the UK. It is the sole supplier of electricity.
All electricity generated is fed to this national grid. Coal/Gas/Wind/Hydroelectric it doesn't matter. There are no separate streams from one generator of electricity that you can request from.
What you DO get is a choice of who bills you.
I'm unsure how these renewable energy certificates work. But it's likely a scam on the generation end that the suppliers will wave at naïve customers to hide the fact that there is zero difference between the electricity you by from eon or British gas, or any of the other billing agents.
https://en.wikipedia.org/wiki/National_Grid_(Great_Britain)
(Edit: added Wikipedia URL)