D Moss has the goods
Basically, there is no such thing as AN id for dealing with the government. There are many roles, some overlapping, some not. Individual departments and agencies have long since sorted out how they identify people for their various taxes, benefits, registers, licenses, etc.
The Cabinet Office, at one time, had a role with the ludicrously overwrought "gateway". Why? Who needs this central id? How will it map to all the separately gathered and verified ids across government? It can't. Let alone supersede them.
Of course, it is tempting to believe the centre of government can identify a citizen role with a single identity. But it can't. To my knowledge the Cabinet Office have been advised on this for at least 15 years without getting a clue. I don't expect that to change any time soon.
But is that even the point? As long as the seat-warmers have their budgets who cares?