The UK can be subdivided in many different ways and the plugin is not able offer all of them. However, with the Pro version we can include custom geojson map sources to generate the maps. That’s what we did here to get the map of Counties and Unitary Authorities in the UK.
We used a dataset from the Open Geography Portal, provided by the Office for National Statistics. We then used some of the tools available to make the map compatible with the plugin. This is the generated geojson file.