(not a lawyer)
It would also require us leaving a whole bunch of international treaties though since "Nationalising a pop singer" kinda breaches a few of them.
The promise could be re-interpreted in some funny ways. For example, Count Binface didn't specify when nationalizing Adele would occur.
They could create another[0] special case in UK copyright law with a minor twist: Instead of eventually entering the public domain, the copyright of Adele's works would transfer to the UK government which would donate any proceeds to a charity that builds affordable homes.
Silly, but it'd mean two promises kept.
[0] https://neverlandofficial.com/discover/peter-pan-copyright/
What’s Europe going to do, send in a deal team to extract Adele?
North Korea? China? Russia? Israel? Gaza? Iran? The USA?
What’s are international courts going to do about it? Gaol Britain? Apply sanctions?
[0] not a lawyer [1] what does “nationalise” even mean?
The shareholders or owners may or may not receive compensation.
Presumably a normal(?) interpretation would be that the British government uses whatever eminent domain law it has and it takes over Adels copyright on all her songs and gives her whatever the fair market value of the catalog is.
But because this is a joke candidate, could be anything.
Etc.