To this point; if you look at google maps satellite view and zoom in/out repeatedly over the UK the yellow line 'road' that doesn't disappear and reload is the construction site of HS2.

If you want confirmation, the easiest bits to "check" are Aylesbury and Coventry. London and Birmingham are too big for the features to stand out.

Here is the official HS2 map: https://www.hs2.org.uk/map/?mapView=9_52.0744_-1.8347