Updated on 08/08/24 by Shads_OVO
It could be you have a looped neutral. See Q and A on this page: The Roll-out of Smart Meters: Smart Metering - The full story.
Relevant Q & A:
What is a looped neutral?
A looped neutral is sometimes used when you have two or more meters next to each other in a property. Instead of being wired up directly to a neutral block, the neutral is wired in series from meter to meter, so they all share the same neutral cable. This means that removing the neutral from one meter will potentially remove the neutral connection from others too.
Why does a looped neutral mean I can't get a smart meter?
Meter installers will not replace a meter which has a looped neutral because it will require every property on the looped neutral to be de-energised. Ideally the neutrals need rewiring, so each meter has its own independent connection to a neutral point. The industry needs to clarify who is responsible for this work.