Cheap or good value?
A lot of 'cheap' companies go to the wall and end up being bought by an existing supplier (most recently EDF and British Gas ended up with another companies customers).
Then there are good value suppliers who charge a reasonable rate and you can actually have a customer/company relationship they answer the phone
Plus there's regional pricing on electricity...so really you want to use a comparison site based on your actual usage once you have an idea which companies you're happy to do business with.