The UN charter (article 23) very specifically says:
The Security Council shall consist of fifteen Members of the United Nations. The Republic of China, France, the Union of Soviet Socialist Republics, the United Kingdom of Great Britain and Northern Ireland, and the United States of America shall be permanent members of the Security Council.
What would happen if the UK split, something that isn’t too far from reality? Would one of the countries retain its permanent security council membership, would both of them get a permanent security council membership, or would the both not have a security council membership?
Also, along the same line of thought, what if any of these other countries goes through a name change? Would they retain their permanent security council membership or have it revoked?