That is a document. Show me reality.

They have 1.5k public repositories here at least: https://github.com/orgs/alphagov/repositories?type=all

Collection of RFCs: https://github.com/alphagov/govuk-rfcs

Open design system: https://design-system.service.gov.uk

And individual departments can/do have their own GitHub org. Eg the Office of National Statistics. Some work I did ~10 years ago can be found there! https://github.com/onsdigital

Next thing we need is for them to host their own Git infra, to avoid dependency on US Github.

Lol remind me who owns Github again?

Why do you think this is substantial? The software built in these repos is not tied to GitHub.

Git is decentralized and using a self-hosted instance of Gitea / Forgejo will give you a replacement for the essential parts of GitHub. GitHub is absolutely replaceable.