I looked at a map of the British Isles and wondered how long the coastline was - the bit around the mainland. It could have been any island - just happened to be the one I live on.
I used to have one of those smart wheel things that you could roll around a line and it would tell you how long it was. That would have been useful. A piece of string would be useful too. But there are quite a few wiggly bits. Not as bad as Norway, of course, but pretty bad none the less.
Walking around the coast with some gadget or other would provide an answer. But that misses out quite a few little ins and outs. Even if I could go around each of the little ins and outs, how far should I go up a river? OK, there could be a rule for that - up to the first crossing.
This gets tricky, though - not the rivers, I am reasonably content about them - the fiddly bits like rocks, stones, pebbles, small pebbles, tiny pebbles. Lumps on pebbles. Even if they all stayed in the same place and even if I could measure around them. The more 'accurate' I try to be, the higher the perimeter becomes. It's not the satisfying outcome of a figure that homes in towards something - these measurements just get wildly bigger and bigger the closer you get to what it is you're measuring.
I had hoped that you could have a triangle (or some other polygon that you know how to work out a perimeter for) that sort of fitted inside our national perimeter and then another that sat just outside it. Logically one would jump to the conclusion that the length we want would be somewhere between the two but no. the damn line in the margin between them wiggles around and gets even longer the longer you think about it.