I believe you are referring to the gov.uk Verify service [1]. It's still in existence, but on it's way out due to poor uptake.

It was pushed agressively at one point by GDS, but despite what it claims about being easy to use, it provided a poor user experience, with very high failure rates for certain demographics.

Universal login, which was the vision for this product for UK government services, is a seemingly appealing proposition but if a high percentage of people can't use it, it's pretty much doomed.

The various UK government departments adopted it sparingly or developed alternatives, which further undermined it.

1 - https://www.gov.uk/government/publications/introducing-govuk...