Well yeah I presumed that not of this world meant aliens. But it probably wasn't, it was very probably something with a reasonable explanation and because you're not an expert on wind dynamics, air pressures, didn't have a good zoomable telescope, etc then it looks unexplained.
Is it more likely that you didn't really have the knowledge to know what a relatively strange but explainable event is, or is it more likely that an alien civilisation/secret Nazis/whatever travelled billions of miles to float a bit above Doncaster uncaptured by many human eyes or the sophisticated RADAR and air defence systems we have in our island nation?
I'm often dismayed by how many people think the second one is much more likely!