The blog they put out alongside the e-mail states:
While we only ever buy 100% renewable electricity and 10% green gas, there is only one price for gas or electricity in the UK. There aren’t separate markets for green and non-green energy sources. The price is determined by what people will pay for energy at a particular time, regardless of the source. So, when the cost of electricity produced by burning gas goes up, so does the cost of electricity from hydro, biomethane, and all other green sources.
Hope this clears things up.
It’s a bit of a silly system and I’d imagine as more and more energy generation in the UK becomes renewable and therefore unaffected by these kind of factors then regulations may change.