It's doesn't matter where British Gas, Scottish Power, or Yorkshire Tea buy their power from - it all comes down the same cable form the same source, and only the distribution network (in my case Western Power distribution) have any influence on that. Indeed, even they have little real influence because the infrastructure is fixed. They can't flick a switch and only pipe Treehugger Power Ltd electricity to your house because you've now signed with them.
No matter what you do the power you use comes form the same source or combinaton of sources, be it coal, gas, solar, wind, or albatross wing beats. It's a paper exercise, not an exercise in physically providing you with power from a more sustainable source.
The most that can be done is to switch in or out strategic level generators - usually gas, because its high output and can be brought on line quickly - to pick up the slack during times of high demand. That's high order stuff, not the sort of thing that can be done to supply an individual customers property.