Tariffs are ALWAYS cheaper if you pay by monthly direct debit.
The fact that the monthly direct debit might be higher than you’d like, and they wont reduce it even if you’re in significant credit, does NOT change the tariff in terms of the actual cost of energy. The direct debit is not “the bill”. Scottish Power will refund credit if you ask, in fact when I was their customer there was even a way to do this via the account portal without having to contact anyone.
To compare if a tariff is cheaper or more expensive than another tariff, you need to compare the unit rate (pence per kWh) and standing charge (pence per day). The amount paid by direct debit is almost meaningless and does not directly reflect the cost of energy.