The Warm Home Discount is a government scheme that gives specific people a £140 discount on their energy usage for winter 2021 to 2022. People eligible are those who receive the Guarantee Credit element of the Pension Credit. If you are under this scheme the money saved switching could be less than the extra discount you’ll get, in which case it's better not to switch.

You can read more about this scheme here: https://www.gov.uk/the-warm-home-discount-scheme/eligibility or ask Plum!