One Login is an "authentication" system, an oAuth provider with identity added on. This means you can prove your identity once (to various levels of confidence, as defined in GPG45 [1]), and use that same verification across different government services.

When people talk about a national ID system, they're often talking about some form of "authorization", i.e. proving that you are entitled to certain things.

There currently isn't a system in the UK that can definitively prove that you have access to every service. For example, even being a British citizen and having a British passport doesn't automatically entitle you to access the NHS.

[1] https://www.gov.uk/government/publications/identity-proofing...