http://www.checkwx.com/wxmain/code/EGCC
Look at the 24 hour trends tab - often the visibility is listed as less than 10 Km - a field test in typical English weather is quite appropriate as you say...:^)
It's not like our visibilities in AZ...
Yet in AZ similar communication methods were used. Mirrors and flags went from quite a few locations. Picket Post Mountain near Superior was so named as one of the locations. I don’t know many others except Pinal Mountain at the head of Dripping Springs Wash. Fort Apache, near White River was a receiving location as well.