This isn't mass psychosis because 'seeing a drone', even when mistaken, is not a symptom of anything. 'Mass delusion' may be closer to what you're referring to.

This isn't mass psychosis because 'seeing a drone', even when mistaken, is not a symptom of anything. 'Mass delusion' may be closer to what you're referring to. But there's a certain aspect of fear or panic in the concept that just doesn't fit here: presumably, nobody would feel panic at the sight of a far-away drone unless they were about to board a plane? In general, I doubt the UK is any more susceptible to such phenomena (cf Americans and their panic at the sight of a turban, or beard). You may also just be profiting from the benefits of hindsight, and throwing in a conspiracy theory for what exactly?