Maps of UK Administrative Areas

Start with County Map or District Map to get a list of unitary authorities, and either counties or districts.
Click through to a map, and a list of wards, and ( if there are any) parishes.
Click through to a parish or ward map

There is a lot of historical complexity!

Some parts of England have 2 tier local government (County and District).
The rest of England, and all of Scotland, Wales, and Northern Ireland have a single tier Unitary authority. e.g. Cornwall is a unitary authority, but neighbouring Devon is 2 tier, with a county council, over several District Councils.
Many English unitary authorities are cities, split off as county level unitary organisations from their surrounding county.
London has a 2 tier structure - the Greater London Authority which is sub-divided into Boroughs, similar to county and districts.
For electoral (not administrative) purposes, both Districts and Unitary Authorities are split into Wards, which elect local councillors.
Separate from Wards are Parishes (though their areas often the same). These are not church parishes, but administrative areas with a Parish Council, which has a local voice on planning matters.
What Brits normally think of as geographic counties (e.g. when addressing a letter) no longer really exist for administrative purposes. In 1972, the historic counties were rearranged, and split into urban and county areas [ see wikipedia]. An example is Middlesex, which no longer exists. The historic counties map shows them.
Not shown, are "ceremonial counties", which are also based on the historic county structure, which are even older areas with a Lord Lieutenant.
Wikipedia explains this bizarre complexity
Some organisations, e.g. the Police, are based on a mix of new and historic county boundaries

Developers

The boundary data comes from OS Open Data in very high (1m) resolution ESRI Shapefile format.
Smoothed (i.e. lower resolution) boundary files are avaiable to download in KML format. Contact me if you need geojson or some other format.

The data is updated periodically. Let me know if a new or changed area isn't correct, and I'll check for an update