It gets called “Butterfly bush” a lot - the BSBI’s Atlas uses it and it’s the preferred name that the NBN Atlas lists.
I find gardeners call it “Buddleia” a lot but it’s probably not the best name for B. davidii specifically as there other species in the genus that can naturalise like Buddleja globosa (“orange-ball tree”) - nowhere near as popular in gardens, and nowhere as well-naturalised though.
Currently there is no English name localized to the UK for Buddleja davidii, so it is displaying the default global English name instead.
Anyone can add a localized name on the taxon page for a species. If there are no other names for the same place and language, it will become the default for that place and language.
If others disagree about what should be the default for that place, though, it is better to flag the taxon for curation and start a discussion first.
Since these discussion are better conducted on taxon flags in iNaturalist, I have opened this flag where the discussion can continue, and am closing this Forum topic.