From what I can gather it seems it is only available to those with a central government e-mail address (e.g. ".gov.uk", ".police.uk" ".nhs.uk"):
That link is probably for government departments (or agencies?) who are being "invited" to use OneLogin to authenticate users. Thus the "Ministry of Dog Fouling" can force you to login should you wish to report and upload a photo of illegal puppy poo.
That way, at a future date, the "Ministry of Citizen Compliance" can be sure that they are issuing the correct person (curtain twitcher) with their appropriate social credits, and the "Ministry of Photograph Tracking" have the serial number of your camera, should you be caught recording things that are not approved by the then government!
Oh, and a OneLogin account is now a LEGAL REQUIREMENT for company directors in the UK (see
https://www.gov.uk/guidance/verifying-your-identity-for-companies-house)! I don't remember the BBC giving that news much publicity?
I too have a Government Gateway account, which i was forced (over five years ago) to create to determine whether and when i would qualify for my state pension. As part of that, i was forced to hand over my mobile phone number, so even then they were trying to slurp as much information as they could about me (even though they already had my NI number).
This creeping centralised computerisation is not for our benefit, and has been slowly going on for years (regardless of the government who are supposedly in power)
