Any meter used for billing of electricity, must be either MID approved or approved under UK National legislation.
Meters already installed, that were approved under UK National legislation prior to October 2006 (OFGEM Approved) may remain installed indefinitely, as long as they are measuring accurately.
For OFGEM Approved meters please follow this link:
https://www.gov.uk/government/publications/schedule-4-uk-nationally-approved-electricity-meters
Any new meter manufactured and approved after October 2006 must be approved under the MID regulations. The Measuring Instruments Directive (MID) is a European Directive (2004/22/EC) that covers a number of different measuring instrument types, including active electrical energy meters. The MID enables a ‘European Type Approval Certificate’ to be issued and the instrument can then be used in any EU Member State. The aim of the Directive is to create a single market in measuring instruments for the benefit of manufacturers and, ultimately, consumers across Europe.
After October 2016 every new meter installed for billing must be MID approved. It is a criminal offence to use a non-approved meter for billing. MID makes no distinction between primary main supply meters and secondary sub-billing meters. They are ALL billing meters.
For more information on MID approval, go to:
https://www.gov.uk/guidance/mid-approved-gas-and-electricity-meters