Dataset - data.gov.uk

Output Area (2021) to County and Unitary Authority (December 2021) Exact Fit...

A lookup between 2021 Output Areas (OA) and County and Unitary Authorities (CTYUA) (Counties, Unitary Authorities, Metropolitan Districts and London Boroughs) as at 31 December...
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You can also access this registry using the API (see API Docs).