In addition to the Stop Lines the other poster has mentioned, there were a number of Auxiliary Units set up. These were undocumented groups with their own arms bunker storing 'terrorist' and assassination weapons whose job was to cause chaos and disruption should we be invaded. These were dotted all over the country, including up in Scotland.
They had all sorts of specialised and experimental weaponry, the links below cover some of the details. My late father was involved in their decommissioning as part of his National Service in the very late 1940s.
https://parhamairfieldmuseum.co.uk/british-resistance-organi...
http://www.pillbox-study-group.org.uk/other-wwii-defensive-s...