While running through the files, I found this comment in one of the industry files:
# This is an anomaly - coal in the UK is now largely imported,
# but there is no concept of "importing" in Simutrans. Therefore,
# a modern day coal mine is needed.
So I had an idea. Have the "modern day coal mine" be a WATER industry, like an oil rig, since coal is always imported to the UK by ship (not by air or rail), representing some sort of floating transshipment dock for international arrival of coal.
Hmm, I thought that I had replied to this, but I must have forgotten (or perhaps it failed to post). This is an interesting idea, but the difficulty is that there are no floating transshipment docks in reality, so this would seem weird and counter-intuitive to the player. Better, I think, would be an import dock (shoreline industry) which simply generates large amounts of coal, and where the boats coming to and from it are simply imaginary. That would, at least, be more consistent, I think.