Not a Brit, not a lawyer...

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/