S'nail-y at the Border - but which one? Traditional Cache

A fun cache in what appears to be a 'no-man's land' between the border signs for both Scotland and England!
Who does this bit of land belong to? What country are we in?

Easy parking with room for 2 cars near the cache, just by the field entrance/England sign.

Whilst down visiting a friend in nearby Kirk Yetholm in 2016, I came across this spot which seems to be between the border of Scotland and England!
From the cache, if you look one way, a great big sign says 'Scotland Welcomes You/Welcome to the Scottish Borders' and about 10-15m down the road another says 'England/Northumberland National Park'.
So... where does this section belong?

I was surprised that no-one had placed anything here already so, thanks to the kind offer of our friend to carry out maintenance, here is a cache that is almost at the border, but not quite!