Hmm... time to put my Ecotricity hat on
I am not a sales person but I do work for them, so have picked up a little info...
Firstly - Ecotricity has two tariffs
1. New Energy - Price matched to your regional supplier, 'standard brown' topped up with well above average green energy
2. New Energy Plus - 100% green with a small premium
Ecotricity's main drive is new build - there is not enough green energy going around, there is high demand from consumers and suppliers (hence the premium on 100% green usually) and not many companies are actively building new renewable sources
at the moment.
If you go for 100% green tariff - you may not be doing much to support the generation of new build (demand generation perhaps
but not actually generation!) and are using up a finite resource...
Ecotricity prefers for customers to go onto their New Energy tariff because this allows more investment in new build. Eventually we might have enough wind parks to supply all our customers with our own 100% green.
Incidentally - Ecotricity also have a microgen reward scheme
currently paying 9p per kWh. In return, you supply Ecotricity with the ROCs awarded to you by OFGEM on an annual basis. Hopefully OFGEM will make the process more user-friendly as more and more people embrace microgeneration at home and work.
Regarding 'Feed-In Tariffs' like Germany - Dale (MD of Ecotricity) has a series of blog posts on this very topic
Does that help? I have tried not to be too biased