A company not subject to price cap as investing in new renewable supply called Ecotricity. The company founder and owner is Dale Vince.
They are investing in allsorts of sustainable enviromental friendly energy production, including biomethane made from grass.
If you want to try Ecotricity use a code to get £50 per fuel credit when you sign up with them, but they seem to often have better sign up offers on their website anyway, perhaps you can use both. If you use Facebook then there is an unused Ecotricity unoffical forum group.
I will also added related links including biomethane below.