The blog post you linked to is a decade old (2011) and mentions using Django and Ruby for Rails for building a "relatively-durable prototype".

The Government Digital Service (GDS) code is on GitHub (1.5K repos) and this includes the frontend code for GOV.UK.

They still use Python and Ruby languages, but it's not clear if they are still using Django and Rail frameworks.

Other languages include Javascript, Go and Java: https://github.com/alphagov