Different tariff prices North & South.

Bulb says they have a single tariff, one for all. However i have discovered that the price of my tariff in London, is different to the price if the tariff a friend has in Sunderland. The DST is the same however the price per KWH are different. My KWH is higher than his for gas, but mine is less for electricity…
So how can it be one tariff for all ???
I await a reply in this matter, many thanks.

This is the same for all suppliers. Different areas have different costs, even within one tariff.

It’s still classed as one tariff, because you can’t switch between anything other than by moving house.