If the messages are end-to-end encrypted, there is no way to do this. A backdoor or other mechanism means it is no longer end-to-end - some third party is involved.

Furthermore, the UK has a long history of such “strict oversight and transparency” hardly existing, when it comes to these matters, with numerous controversies with police and other agencies abusing their access to systems.

It is worth noting, however, that WhatsApp messages can generally be accessed with such a backdoor anyway. The majority of users backup their messages to iCloud or Google Drive, encrypted, with WhatsApp holding the key. Consumer tools already exist that can access these encrypted backups, and fetch the key from WhatsApp with nothing but a text message. A warrant to Google/Apple for the backup is all that is needed, apart from the small % of users who have enabled the new E2EE backups.