How Long Is the Coast of Britain? (1967)
jstor.orghttps://gsp.humboldt.edu/OLM/courses/GSP_510/Articles/Mandel...
Link to a pdf file that you don't need an institutional login for.
I did an activity in a basic maths class based on this paper years ago. Each student had an A3 map of the main island of the UK. Some set their compasses to 5cm radius and counted the number of radii around the island. Others tried 2.5cm, and 1cm and half a cm. Worked ok, good lesson.
Depends on your measurements. If you measure with 1 cm it is longer than if measure with 10 cm.
Obviously too long to defend against rubber dinghies.
There is a lot of coastline and not that many police/coastguards. In fact we have been closing down the coastguard stations since satellite tracking of commercial shipping became the norm.
(I come from a part of the UK that was notorious for smuggling, wrecking and other forms of piracy).
Infinitely long. You can't trick me with the coast paradox