Quite similar to Transport New (actually better), but not an exact match. Maybe a custom version?

I looked into the css of the site you linked to and there’s mentioning of a font-family named GDS (obviously for Government Digital Service). So, like I suspected, it’s custom.

Read about it here.

Thank you, and great find. Despite Googling around for a while I couldn’t find anything. I’m really pleased to see the UK government using beautiful web fonts on their web presence. Thanks again.