Sorry if I sound a little ignorant but, as an outsider, I’m not entirely sure what you mean when you say “GOV.UK content”.
“GOV.UK” to me means “all government websites”. So is this an API that will encompass the Met Office, HMRC, TfL, etc? Or, does “GOV.UK” mean something more specific to you that isn’t entirely clear to me?
And “content” to me, in the context of a website (which is, I think, the context here) means “everything on the pages”. So… well, does that mean everything (text, images, statistics, etc) on every [subdomain].gov.uk website? That seems rather broad so, again, maybe you mean something more specific by “content”?
I know you do your best to keep things clear, and to avoid jargon, so I hope you can clarify this.