"It should have at least been the same conditions as the Scottish independence referendum."
Actually that also was a FPTP result.
However.
Only people who lived in Scotland were allowed to vote. No post in's from France/Germany/US/British Cayman Islands etc.
AFAIK no restrictions on nationality.
16YO's could also vote, and it was expected they would be excited to leave. Except they weren't.
Point was the people who voted were living in the country now, not some picture of "Scotia" they had in their heads when they left 20 years earlier to retire to their pool side in some offshore tax haven.
It could also be argued the campaigns were more factual and less emotional than the EU referendum, less clouded by the British Empire, little England mentality. So people voted with their heads, not their hearts.