Scotland did the referendum in agreement with British Law and Westminster pairlament. This, note there is no British Constitution that forbids them to do referendums.
About Flakland Islands I don't know, so I don't have an opinion.
I imagine it's dependent on boundaries not individuals. A nation, an island, a state, or a city might ask for independence, but not a person or a group of people. In the US we even have semi-independent spaces like reservations and embassy buildings where a single complex or structure is considered separate from the nation that surrounds it.
a lot depends on the outcome. you can bet that the british empire didn't "let" the colonies secede, but they did anyway, persevered, and eventually the british "let them" by way of not pursuing the war.
and so, here we are. the same could happen with spain, or a state in the US, or w/e. winners tend to write the history books, as well as perspective on past events.
To achieve it though they will have to convince the whole world that it is true and more important then anyone's relationship with Spain