My best guess is:
The Falkirk Wheel, in Scotland:
On a normal working day, in unicorn land.
The Falkirk wheel is a boat lift that operates throughout the week in Scotland. Scotland's national animal is the unicorn, so Scotland is the best real world place to call "unicorn land"
Two opposing ways of the sub, I shall demand.
The Falkirk wheel allows boats to pass both ways, between the higher canal and the lower canal, at the same time. Admittedly, it's a bit shallow for "subs," but some of the tour boats are sealed all around and look like submarines.
At the head of a hill, I will thus stand.
From the picture above, you can see that the wheel connects directly to a decent sized hill.
If it's not this, then I still
Think that it's somewhere in Scotland, perhaps near a naval base, with actual submarines, or a city, with subways (though Edinburgh doesn't have an underground system, as far as I can tell).