You're being selective about what international law actually says about self-determination here.
In reality, self-determination is subordinate to the right of unitary sovereign states to maintain territorial integrity.
The only times self-determination takes precedence is in cases of colonialism and oppression by the sovereign parent state.
"In a UN context, the right to self-determination in its external shape is applicable to people (not to national, ethnic, and religious minorities, whose rights are recognized in Article 27 of the ICCPR, 1966) or to the nations in the cases of: a) a colonial context; or b) in a situation of any foreign domination or occupation."
"Today, it may be concluded that international law bestows on all peoples the right to self-determination, but that the right to external self-determination, exercised through remedial secession, only applies in extreme circumstances, to colonized and severely persecuted peoples."
Scotland, Wales and Northern Ireland are NOT colonies but INTEGRAL parts of the UK. And they are definitely NOT oppressed by the UK.
So, under international law they have no right to secession. But this does not mean they don't have a right to self-determination. However, that self-determination doesn't appear the way you think it does!
As long as unitary sovereign states provide some degree of power to their regions they have the entitlement in international law to protection of their territorial integrity. In the UK, that means devolution to Scotland, Wales and Northern Ireland which is the extent of self-determination they're entitled to under international law.
International law makes it very clear that territorial integrity takes precedence over self-determination (except in two circumstances) and it further states there is no democratic right to secession.
Secession is not a democratic right
"Currently, among the constitutional laws of the world, only two states carry the right of secession in their constitution: Ethiopia, and the Federation of Saint Kitts and Nevis."
- My Country, Europe
“In the Federal Republic of Germany, which is a nation-state based on the constituent power of the German people, states are not ‘masters of the constitution’.
Therefore there is no room under the constitution for individual states to attempt to secede. This violates the constitutional order.”
- Ruling from the Constitutional Court of the Federal Republic of Germany.
"The Constitution is based on the indissoluble unity of the Spanish Nation, the common and indivisible homeland of all Spaniards; it recognizes and guarantees the right to self-government of the nationalities and regions of which it is composed and the solidarity among them all."
- Section Two of the Spanish Constitution
"France shall be an indivisible, secular, democratic and social Republic."
- Article One of the French Constitution
"The Republic, one and indivisible, recognizes and promotes local autonomies; implements in those services which depend on the State the fullest measure of administrative decentralization; accords the principles and methods of its legislation to the requirements of autonomy and decentralization."
- Article Five of the Italian Constitution
"It is clear that international law does not specifically grant component parts of sovereign states the legal right to secede unilaterally from their "parent" state....
The various international documents that support the existence of a people's right to self-determination also contain parallel statements supportive of the conclusion that the exercise of such a right must be sufficiently limited to prevent threats to an existing state's territorial integrity or the stability of relations between sovereign states...
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 own internal arrangements, is entitled to the protection under international law of its territorial integrity."
- Canadian Supreme Court ruling on the issue of secession
"I pledge allegiance to the Flag of the United States of America, and to the Republic for which it stands, one Nation under God, indivisible, with liberty and justice for all."
- United States of America Pledge Of Allegiance
"The answer is clear. If there was any constitutional issue resolved by the Civil War, it is that there is no right to secede. (Hence, in the Pledge of Allegiance, ‘one Nation, indivisible.’)”
- Supreme Court Justice Antonin Scalia in 2006
“Article 52 of the Constitution is a unitarian structure, indivisible, indecomposable."
Dean of the Brazilian Supreme Court, Judge Celso de Mello
"WHEREAS the people of New South Wales, Victoria, South Australia, Queensland, and Tasmania, humbly relying on the blessing of Almighty God, have agreed to unite in one indissoluble Federal Commonwealth...."
- Preamble to the Constitution of the Commonwealth of Australia
You are, therefore, being unreasonable.