It's called the National Grid ESO scheme and you sign up through your supplier when it's open. However, some suppliers take a cut of the ESO payment. Alternatively, you can sign up to Uswitch Utrack (https://www.uswitch.com/mobile-app/), which passes 100% onto the customer. (Disclaimer: I used to work at Uswitch.)

octopus energy seems to be the best for energy i have seen so far in the uk alongside their smart tariffs https://share.octopus.energy/umber-squid-619