This is Argo (the first Argo, the UK one, founded 1951): https://musicbrainz.org/label/d1013f66-e931-4083-94a0-4c30b6853167/
See the following article for a potted history:
My understanding is that it was originally an independent label, then purchased by its distributor Decca in 1957. It continued to operate as an independent label under the direction of its founder Harley Usill.
Currently, the label has the following relationships:
Parent label: Polygram (1980-1988); Decca (1990-1998)
Imprint of: Decca (1957-1980)
I don’t think this is correct. The Decca relationship was constant throughout, e.g., here’s a CD from 1986, Decca colours and “The Decca Record Company Ltd” prominently stated: Haydn - Barbara Bonney • Anne Howells • Anthony Rolfe Johnson • Stephen Roberts • London Symphony Chorus • City Of London Sinfonia • Richard Hickox – Nelson Mass / Nelson-Messe (1985, CD) - Discogs. Note this does not represent a change from the LPs of the 1960s and 70s, as those LPs also had the same language
The Polygram relation seems tenuous, apart from Polygram’s ownership of Decca. Any Decca releases at this time, by this definition, could be considered Polygram, but it’s not the primary label. (More here from around that time: https://www.discogs.com/label/645373-Argo-2?sort=year&sort_order=asc&page=38)
Here’s an example of the relaunched Argo, same language relating it to Decca: John Harle – Michael Nyman: Where The Bee Dances / Gavin Bryars: The Green Ray / Mike Westbrook: Bean Rows And Blues Shots (CD) - Discogs
The relationship doesn’t really stop in 1998. There are a number of examples of reissues. A good example is this massive 2016 100-disc box set: Shakespeare - Decca. Another example is this 2010 compilation: We Will Remember Them (2010, CD) - Discogs. There are other examples of reissues in Japan of Argo material post-1998 under Decca auspices (Garrick Sextet – Promises (2003, Paper Sleeve, CD) - Discogs; https://musicbrainz.org/release/1b90a7c3-b9cc-4675-aca2-f2ae498823fd).
At minimum it seems to me that Argo should be a subsidiary, not an imprint as it was operated as an independent label up to the end of the 1970s. It is then used for reissues thereafter, with select new releasaes, largely analogous to what happened to labels like Verve.
I’m not sure how to express the 1990-1998 relaunched Argo, or if we should, but it seems that an imprint would be much more appropriate here as Argo was no longer autonomous within Decca by this point. I haven’t seen other examples of labels’ relaunched operations e.g. Verve or Impulse expressed as relations.
The Polygram relation does not seem to be tenable, as I can’t find any examples of releases of Argo without the Decca link. Polygram may have directed some of these issues but the label is clearly Argo.
I was going to enter this as edits but the difference between an imprint and a subsidiary is not clear to me, so I’ve written what I would have written as an edit note her.