Last paragraph doesn't sound like it's ok either; It must surely be incumbent on the UK to seek to amend the default positions which apply in the event of no further agreement, rather than to exercise a unilateral right of withdrawal which could not be justified under Article 62 VCLT
The amendments to UK law are arguably not in breach of international law. The VCLT does not strictly apply to any treaties between the EU and the UK.
https://www.blackstonechambers.com/news/article-62-vienna-convention-law-treaties-foreseeing-unforeseen/#:~:text=The%20VCLT%20makes%20clear%20that,within%20the%20international%20organisation%20itself.