It’s a speed limit, not a target.
This is true - but its incredibly frustrating. And whilst we are able to quietly seethe and then argue about how annoying it is on the internet, I wonder how many hot headed people lose their rag and end up performing ridiculous and unsafe overtakes they wouldn't have done if everyone was driving at the appropriate speed for the road? You only need to see some of the moronic driving that goes on when somebody has to sit behind a cyclist on a B road for more than 20 seconds