I remember they had a design document ( Something similar to [1] [2], but not the exact one I was looking for ) where they list accessibility as their number one priority and a very long list of accessibility requirement. Simply because Government Web Site needs to be accessible by everyone. That means there are certain fancy things and colour scheme they cant do.
I wonder if the stack for Gov.UK are the same as their Petition Site [3] or was it based on something else? They have an Open Source Github account [4] for Government Digital Service which list everything they have open sourced.
[1] https://www.gov.uk/guidance/government-design-principles
[2] https://design-system.service.gov.uk