The Government has rules about road lettering. The typeface is called ‘Transport Medium’. The letters have a standard width, for example an A is 544mm wide and an F is 476mm wide.
Without distortion the letters are 560mm high. For roads with a speed limit of 40 miles an hour or lower they are stretched to 1,600mm high. For roads with a speed limit over 40 miles an hour they are stretched to 2,800mm high. In this case the height of the letter is multiplied by 5.