You can assess whether only one value or more-than-one is on the sheet. Based on that you can decide what to display.
I'm opening your workbook now.
See attached. In SHeet 7 I added a calc (displayed in the title) that tells you how many cities are on the sheet.
You can make a filter that says to display if [Cities on Sheet] =1.
(See Sheet 7(2) for that.)
Important note: This calc is NOT counting what is clicked in the filter. It's counting what is on the sheet. We don't have access to the actual filter selections. So I changed your filter to be "only relevant" display. That means when the country is England, you see only cities from England. If you have all the cities in the filter, you can click a bunch from Germany and it won't increment the [Cities on sheet] calc because there will not be german cities on the sheet.