If we are to accept the validity of demanding that skeptics prove that UFOs are not alien space ships, one could also ask them to prove that they are not smurfs on a joy-ride, or evil robots from the future. Following Nick Pope's logic, it would then be reasonable to assert that smurfs or robots exist based upon a lack of proof to the contrary. What Nick Pope should realize is that it is very difficult to prove a negative. For example, a believer in Santa Claus might challenge skeptics to prove that reindeer cannot fly. No, all he has proved is that these particular reindeer could not fly, or chose not to for some reason.
Negatives are prohibitively difficult to prove.
And so, it is the burden of the claimant to prove his claim. In the case of UFOs the skeptic would assert that the object was unidentified. This claim is proved by default, because nobody knows what it was. The claim of alien visitations is not proved by default. Ignorance of what something was is not proof that it was built by aliens.