Many people who are interested in low energy homes are in fact less profligate in their use of energy than most just because they are aware of the cost to pocket and planet.
But the way that the energy companies organise their pricing structures means that the first units used cost more, while energy used above a certain threshold becomes cheaper. This is rather unfair if you are a very low user, so it is worth shopping around to see if you would pay less with a different energy company.
I understand (but have not fully checked the details) that the Co-op has a flat rate deal that may work out cheaper for those who use less energy. You can also check on the website for details of the energy mix to get an idea of how "green" their energy is.
Other companies who focus on renewable energy over fossil fuels are Good Energy and Ecotricity