What are the roadmap stages?

You can look through the Covid-19 Risk Assessment for the risks at each stage: https://docs.google.com/document/d/1nNr2gtYkQWiJlv9DPU8j-CKOmmUT7EOC9dWpscPZmZc/edit?usp=sharing

The Government website also has an outline of the current roadmap model: https://www.gov.uk/government/publications/covid-19-response-spring-2021/covid-19-response-spring-2021-summary