The cap on what mobile operators can charge each other was agreed as part of the Free Trade Agreement between UK and Norway, Iceland, and Lichtenstein, which entered into force on 1 September 2022.
The mobile roaming cap is the first of its kind in an FTA.
More information (in Norwegian): Frihandelsavtale med Storbritannia sikrer fortsatt Roam Like Home - regjeringen.no
See also: Mobile roaming cap to benefit Brits abroad - GOV.UK (www.gov.uk)