Mike Westbrooke Metropolis album Cover

hello all, hope I'm posting this on the correct forum.

One of my all time favourite albums is Mike Westbrook's 1971 album 'Metropolis'. Whenever I play it I always end up gazing at the album cover where an empty dual carriageway climbs over a rise and disappears into an evening lit city ahead. very ethereal, particularly when playing the last track on the album. There is a sign post in the foreground pointing, it seems, to London so I'm thinking it isn't London that we can see ahead but somewhere outside. Its too fuzzy to discern any more.

I'm wondering if anyone might have ideas as to where this location shot was taken? I have tried several times to track down the graphics company involved but to no avail. It was, after all, rather a long time ago. The cover was printed in Slough so I guess that might be a possible. I might just have to end up driving every dual carriageway in the home counties to get this obsession out of my head otherwise!