The short answer is that it costs more to keep the energy network running in some parts of the country than in others.
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The main reason this cost changes based on your postcode comes down to your local Distribution Network Operator (DNO). Britain is carved up into large regional chunks, and each local DNO splits its running costs among everyone who lives in that specific territory.
These regions are massive. For example, the North Wales and Mersey region lumps the densely populated city of Liverpool together with the rugged mountains of Snowdonia. Everyone in that vast zone pays the same regional average.
Here is what drives those regional costs up or down:
Population density: In a densely populated place like London, the local network costs are shared among millions of households packed together, bringing the individual cost down. In areas with fewer people, the cost per home is higher.
Terrain and weather: It is physically harder, and therefore much more expensive, to maintain power lines in areas with brutal weather and tough geography. Running cables across the Scottish Highlands or Welsh mountains costs significantly more than maintaining lines across the relatively flat east of England.
The "weird map" effect: Because DNO boundaries are so large and diverse, city dwellers end up subsidising higher maintenance costs if they have remote, difficult-to-reach areas within their shared region.