https://www.joe.co.uk/news/brits-in-disbelief-as-new-refillable-drinks-ban-implemented-across-uk-508201

An original consultation took place during 2018 as part of the previous government’s Child Obesity action, and legislation was finally passed in Parliament in December 2021.

The rules only came into force on Wednesday (1 October 2025).

The legislation was actually passed under the Johnson government:

https://en.wikipedia.org/wiki/Second_Johnson_ministry

I suppose that Labour could have passed a law canceling implementation, though.