Can Scotland secede without permission from London, from the standpoint of international law? There's a thing called "the right of a people for self-determination". I wonder whether it's applicable to Scotland. Could they carry out a new referendum regardless of what Boris thinks on the matter?
There are two major relevant judicial decisions: the International Court of Justice's advisory opinion on whether Kosovo's unilateral declaration was in accordance with international law, and the Supreme Court of Canada's opinion on whether Quebec had the right under international law to unilaterally secede.
Judicial opinions aren't terribly common, because questions of secession are usually handled through force of arms. Although the Canadian decision was made by a national court, it's considered a significant work of reasoning and is often cited on this issue (for instance, it was heavily cited in briefs before the ICJ).
The ICJ held that Kosovo's unilateral declaration of independence was in accordance with international law. However, this did not mean that Kosovo could unilaterally secede. As the ICJ noted, they were not asked what the legal consequences of the declaration were and they were not asked whether Kosovo was independent. They were only asked whether making the declaration violated international law. The ICJ found that in general international law does not prohibit unilateral declarations of independence. In particular situations they might violate other rules of international law (e.g. if they're tied to an unlawful use of force or violation of peremptory norms), but in general a unilateral declaration of independence is not illegal. Again, though, the ICJ took pains to emphasize that they were not making any decisions about the right to secede. The case was only about announcing that a country was seceding.
The Supreme Court of Canada's ruling, in contrast, was about whether actual secession was allowed. The court found that it was not, under either Canadian or international law. While international law creates a right to self-determination, that right typically only justifies secession in colonial empires. In general, a people is meant to achieve self-determination in the state they reside in. When a people is fully integrated into the life and governance of their state, they are not being denied self-determination. They might not always get what they want, but if two people disagree then someone is always going to be disappointed. The court held that
A state whose government represents the whole of the people or peoples resident within its territory, on a basis of equality and without discrimination, and respects the principles of self‑determination in its internal arrangements, is entitled to maintain its territorial integrity under international law and to have that territorial integrity recognized by other states.