I respect the UK dot gov designers so much because they understand this: there's no competitor or open source alternative to the government. There's just the one place to get these services (which are not only necessary but mandatory), and so the standard of the experience must be impeccable.
Public digital services need to be *so much better* than anything the market offers. You can't switch. You can't get fed up and choose to pay a bit more at Ocado than Sainsbury's because the UX is better. You can't opt out of public services because you're digitally excluded.