There is very heavy traffic on both sides, so it wouldn't be unusual for the actual speed to be 50mph, 40mph or even less. The second nearest car is braking, and there's a "Queues Likely" sign a little further -- I found the place:

https://www.google.com/maps/@53.3015568,-2.722796,3a,75y,235...

StreetView gives a much better impression of the chevrons; the Alamy photo has an unusual choice of lens. The "M56 B 45.0" [km] milepost is way ahead of the chevron sign.