No, it isn't.

The closest that UK copyright law gets is that there's an exemption for photographs of "sculptures, models for buildings and works of artistic craftsmanship, if permanently situated in a public place or in premises open to the public". (See section 62 at https://assets.publishing.service.gov.uk/government/uploads/... .)

But Banksy's pictures are just straight "artistic works". There's no need to class them as "works of artistic craftsmanship" and I don't believe any court would do so.

The infringers are either deliberately misreading copyright law or simply ignorant of it. I suspect the latter.