as far as I am aware all the "big six suppliers" offer social tariffs for customers on the correct benefits To qualify for a social tariff the customer must be over 60 years of age and in receipt of Passport Benefits such as, Attendance Allowance, Council Tax Benefit, Disability Living Allowance, Child Tax Credit (where income is less than £14,600 per annum), Disabled Persons Tax Allowance, Housing Benefit, Income Based Job Seekers Allowance, Income Support, Pension Credit, Working Tax Credit (where household income is less than £14,600), Industrial Injuries Disablement Benefit or war Disablement Pension.
this would then give them the cheapest rate from that supplier regardless of payment methord.