You can get INSPIRE polygons from https://use-land-property-data.service.gov.uk/datasets/inspi...

Either download the GML or view it on the map. This gives you a number which you can pump into https://eservices.landregistry.gov.uk/eservices/FindAPropert...

And the spend £3 buying the deeds for the property. One look at the coverage in your local area shows the type of properties available, for example Parliament Square (North and South), or tiny bits of rock off the edge of the Isles of Scilly, as well as more pedestrian locations like "Flat 1-14, Grosvenor Court, 55 Upper Grosvenor Road, Tunbridge Wells, TN1 2DY"

A lot of unregistered land, certainly in towns, are roads and railways that have been there for over 100 years