For our electricity, we have a REGO certificate (Renewable Energy Guarantees Origin) for every MWh we supply.
For our gas, we have Renewable Gas Guarantees of Origin (RGGOs).
The purpose of the certificates is to prove to the final customer that the energy was produced from renewable sources.
The electricity certification process is audited annually by Ofgem the regulator of the Energy Industry. And our gas is certified by the Green Gas Certification Scheme (and our EKO Energy Tariff is additionally certified as sustainable, by an independent ecolabel, EKOenergy.org)