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
Collection of RFCs: https://github.com/alphagov/govuk-rfcs
Open design system: https://design-system.service.gov.uk
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.