I guarantee there was a "reason"(add your own definition of the word reason) for it @ChrisJ if it was on a "smart" motorway, having had to attend a motorway awareness course myself recently.........even possibly one of these......
a) a problem up to 20 miles further on and they're slowing traffic from getting there so that it's sorted before you arrive
b) general volume of traffic triggering sensors to slow everyone down for "safer driving" lol
c) testing of the equipment (at the most inappropriate time for you)
d) upping the figures and therefore the revenue stream to justify 3 people presenting a course.......
e) making motorways safer
I had 50 miles of 60 mph for no apparent reason on the M1 through Yorkshire down through Nottinghamshire very late at night and ended up triggering a speed camera as i couldn't maintain concentration through boredom and a touch of pissedoffedness on a long downhill stretch.....