Ministry of Transport and Civil Aviation and the Central Office of Information, The Motorway Code — On the Motorway (1958)
“ DO NOT OVERTAKE ON THE LEFT. 17 Before pulling out to your right into a traffic lane carrying faster moving traffic, watch out for and GIVE WAY TO TRAFFIC IN LANES TO YOUR RIGHT. USE YOUR MIRROR to make sure that the lane you will be joining is clear for a long distance behind, and bear in mind that an overtaking vehicle may be travelling much faster than you are. Give a clear signal of your intention WELL BEFORE you change from one lane to another. ”
