The annexation of Crimea was a based on a referendum in 2014 that breached international law on the issues of self-determination and right to territorial integrity. See section 73 of the Ukrainian Constitution.
I'm sure you don't realise that there is no right to secession/independence under international law. And this is democratic as well as legal.
The Crimean referendum was as illegal as the Catalonian referendum.
The UN voted to declare the Crimean referendum illegal. Mainly because Russian troops were already in Crimea beforehand which means it isn't a freely cast vote.
Additionally, Boris Johnson had no mandate at all as it was a Conservative Party manifesto. We elect governing parties to power. Not individuals.
You're digging a hole for yourself due to a lack of political knowledge.....