[set fair (OP)]

Googling gives 17819 km according to the ordinance survey. Anyone know how they calculated that figure? One can readily see that if you used a ships rope, pressing it in as tightly as possible you would get a shorter length than if you used fishing line. I would guess using the fishing line might double the length compared to the ship's rope, although 10 times longer wouln't surprise me.

Anyone have a reasonable guess to what you would get using a tellurium string one atom thick... also got this after Googling 'thinnest possible string'?

It is one of those quiz quetions that always annoys me, as is what is the largest island.

[alancalverd]

Classic fractal problem! If you measured around each grain of sand above the spring high tide mark, it would probably run to a billion km, and a bit more if you took the low water mark!

The Ordnance Survey length is probably the circumference of the smoothed high tide contour that appears on the OS maps, but why not include non-tidal river edges too? That will easily double the length.

The good news for the English taxpayer is that at least half of the tidal coastline is in Scotland, so Scottish independence will halve the cost of the inshore Navy and Coastguard, and give us an easily defensible land border across the narrowest part of the mainland. 

As to the largest island, it all depends on what you mean by an island! Politically, it's obviously Australia as being the largest piece of land entirely surrounded by sea and governed from a single location. Antarctica is twice the size but arguably not a single country.

[Petrochemicals]

Fractals as Alan says is the obvious answer, I would not be surprised though if the UK coast line was measured by surveyors using distance wheels, we are a highly scrutinised bit of land.

[vhfpmr]

Classic fractal problem!

The skydiver's nightmare:
https://www.youtube.com/shorts/5GTW2SDYGhY