Motorway speed signs

Driving london bound on the M20 this morning the signs in the central reservation where flashing 50mph,i slowed to that speed but seemed to be the only one ,the signs where showing 50mph for about 8miles

Question .was i right for slowing down to the said speed or was i getting in the way of those who did not and carried on at 70plus

It turned out to be no problem on the motorway .but how do you know that????