Interesting. What are those lanterns to the left?If you google "M4 in 1970s", the first image which is listed appears to be a frame from this footage as used in the James May programme. The whole clip is on Youtube and is listed as: "1970s POV from Car Driving Away from West London towards M4 motorway." (sorry - I don't know how you set a direct link to this clip). It has been uploaded by "thekinolibrary".
The start of the film shows central res' catenary lighting, followed by the concrete columns with galv steel twin outreach brackets as seen on your pic. At first glance, these lanterns look like Alpha 5s, however looking at the portion on the video, they look more like GEC Clearmain Z9504s.
The slip road lanterns also seem to be mounted on concrete columns, but do not have steel outreaches. At first glance, I would suspect the lanterns are 140W sodiums, however the scale doesn't seem quite right in relation to the other columns. Although it would be common to have both mercury and sodium together back then, my other suggestion is that these lanterns might be GEC Z8524s. Interestingly, if you follow the video near to the end, an adjoining sliproad seems to be lit by flat glass Z8520s on steel columns with long outreaches.
The general problem we have on footage like this, is that there is a degree of moving image distortion, which tends to make the lanterns on the sliproad more tricky to identify. This distortion may be due to the film quality and to some extent by the curvature of the car's windscreen.
That said, this film is certainly worthwhile seeing.