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@coldharbour
Looking back in the Streetview history (click the date in the top left box and then play with the slider at the bottom of that box):
In all the pics until July 2012 it had an old style illuminated plastic box (in the middle and on the other side of the road too). https://goo.gl/maps/ignToivUzcq
The next pic is from September 2014 and there are only new style flappy boys on the central reservation: https://goo.gl/maps/k6pMzqDZFxC2
Looking from the other direction doesn't help. From a side road narrows it down to sometime in or before June 2014 for the works: https://goo.gl/maps/APuZG3u1xZH2
So (obviously) sometime between those photos they replaced the old style ones with the new style ones.
What it doesn't answer is whether the'd replaced the ones at the sides with flappy ones or did away with them at that time.
I'd get as many photos of you can of the current set up and other set ups on the same road (especially if they do have ones at the sides), including the dated screenshots from google maps.
https://www.google.co.uk/maps/@51.4120385,-0.1594554,3a,75y,67.03h,79.94t/data=!3m6!1e1!3m4!1sotY8QQ7hnhGU-bxRdGVE4w!2e0!7i13312!8i6656