It’s supposed to be enforced by average speed cameras. With some systems, if you entered the timed gauntlet in one lane, then emerged in another it could ‘lose’ you, though they may have fixed this now. I just stick to the limit and was recently rewarded by being overtaken by a Tesco HGV that ripped off my door mirror. If I had been allowed to legally travel at my own speed, that’s one ‘accident’ that would never have happened.