Switching to a “green tariff” may not bring new investment in low carbon generation and the companies who do offer the greenest tariffs (e.g. Ecotricity, Good Energy, Green Energy, Octopus) are not big enough to build the kind of generation we need to properly green the grid (large on-shore or offshore wind farms, 100MW+ solar farms). These are funded through government support (currently the Contract for Difference, CfD) and we all pay for this support through our energy bills already - whatever the tariff we are on.
Agree that if you have solar PV, small wind or small hydro then you are generating 100% green energy for your own use. Another route is to buy direct from a green generator on your local distribution network.
I refer you to my disucssion with Tom on the carbon calculator and green tariffs: