“The UK is said to have more administrative boundary changes per year than the rest of the European Union put together.
Electoral wards/divisions are the key building blocks of UK administrative geography. They are the spatial units used to elect local government councillors in metropolitan and non-metropolitan districts, unitary authorities and the London boroughs in England; unitary authorities in Wales; council areas in Scotland; and district council areas in Northern Ireland.”
Office for National Statistics Web Site
Each country in the UK has a slightly different structure and the picture is further complicated with Unitary Authorities, London Boroughs, Metropolitan and Non Metropolitan Districts.
The wards and districts overlay grabs the wards/divisions associated with the post code at the time and the grouping immediately above ( London Boroughs/Non-Metropolitan Districts/ Unitary Authorities and Metropolitan Districts). You can then select, analyse and explore your contacts based on this hierarchy.