This concept of I am only using 100% renewable is not actually correct, ALL the electricity comes from a pool of sources and is fed into the national grid, so your electricity has a mix of sources, nuclear, wind, gas and coal fired etc. Basically whoever you pay for your gas and electric, it all...
but Tesco own those, the network is maintained by a different company than your gas or electric supplier, either UK Power Nrtworks, National Grid etc. The suppliers have to pay those companies to maintain the meters, pipes, pylons etc