The (capped) gas tariff has 14 regional variations for unit cost combined with non-regional standing charges.
Prepayment:
Standing charge: 37.51p per day
[Northern] Unit rate: 10.72p per kWh
[London] Unit rate: 10.97p per kWh
[Average] Unit rate: 10.82p per kWh
Direct Debit:
Standing charge: 28.49p per day
[Northern] Unit rate: 10.20p per kWh
[London] Unit rate: 10.51p per kWh
[Average] Unit rate: 10.33p per kWh
All above figures include VAT at 5%
Paying each month after getting a bill is typically 6% higher than DD.
https://www.moneysavingexpert.com/utilities/what-are-the-price-cap-unit-rates-/
I am currently being charged around £3.40 per day on a DD tariff; the same usage would cost around £3.62 on prepayment.