Make websites with R Markdown styled with the GOV.UK Design System. See the website.
If you are developing a Shiny application, then look at https://github.com/moj-analytical-services/shinyGovstyle.
Use install-govuk-frontend.sh to download, modify and compile the GOV.UK Design System to get the css files for the templates.
The sassc command-line tool. If you can’t get that to work, try the sass package by RStudio, which wraps it.