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.
There was a recent history hit podcast about this. Apparently their first job on invasion was to assassinate anyone who knew where their arms were stored, including local British officials and ex-members of the unit.
Given how secret it all was, I wonder how many of their arms and explosives caches are still hidden in the British countryside.
Rommel's Afrika Korp did very well against the British and the fixed defenses the British had built. The Korp was supplied entirely by sea and air.
The Korp eventually was overwhelmed because:
1. The British were far better supplied with troops and equipment (also shipped in)
2. The Korp used Enigma, which the British had cracked. With that, the British knew where every German supply ship was and when, and sank most of it. They also knew Rommel's battle plans in advance.
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...
https://www.staybehinds.com/