@riotlady The UK's byzantine energy market isn't my area of expertise, but as far as I can understand it works like this.
Getting electricity into your home or business is divided into four parts: the generators who pump electricity into the grid (i.e. power plants, turbines, solar panels, etc); the transmission (the high voltage pylons which send electricity from the generation points to the local substations); the distribution (from the substation to the end user - homes, businesses etc); and the suppliers or retailers (the people to whom you pay your bill).
You sign a contract with a supplier and they, in turn, will have contracts with one or more generators to take delivery of a certain amount of electricity in any given period, typically a half hour at a time. Your "green" supplier may only have such contracts with a renewable generator. All fine so far.
The problem is that if your supplier's renewable generator(s) relies on solar and wind, and you turn on your airconditioning, charge your car, and run a 2,200w vacuum cleaner at the same time your DD is using her hair dryer, and this all takes place late on a hot still night when there's no wind or sun, the renewable generator cannot meet the demand which your green supplier has contracted for. Your green supplier therefore must buy additional electricity in real time through what is effectively a clearing system from other generators putting electricity into the grid. By the same token, if you are out at the beach on a sunny, windy day then you cannot use the electricity which your supplier has bought from its contracted generator(s) for those half hour periods when you'd normally be using it, so they sell it to other suppliers. They all then sort out the money later on their side through a 'balancing payments' system. National Grid ensures that supply and demand is roughly balanced by shutting down or activating surge capacity as and when needed.
There's no way you can distinguish between surge capacity coming from a gas-fired power station in East Anglia, nuclear power coming from France, or stored hydro-electric power coming from Dinorwig in Wales, all of which goes into the grid at peak times and ends up in your home even though you only pay money to the "green" supplier which only has contracts with renewable generators. The transmission and distribution costs are split between the retail suppliers (which tacks them onto your bill) and the generators (which tacks them onto what they charge the suppliers).
As far as I can tell, if I could meet the regulatory standards there is nothing to stop me setting up Otherpeoplesteens Electricity plc, buying electricity from Back of Beyond Windfarm Co Ltd and selling it to you as renewable energy. The thing is, I could also buy electricity from Old Smoker Coal Generation Company, then buy carbon credits - which are actually quite cheap - from any old company which has them spare, or on the open market through one of a number of emissions trading exchanges, and still sell 'carbon neutral' electricity to you.
The other issue, of course, is that electricity doesn't account for anywhere near all domestic energy use. Most people in Britain use natural gas for central heating and hot water and while there is some renewable biogas getting into the system, gas is a fossil fuel by definition to all intents and purposes. Anyone peddling renewable gas or duel fuel tariffs is either generating said gas from their arse, or talking out of it.