I don't always drive at the speed limit, even if conditions dictate that I could. If I'm just out for a drive rather than making a specific journey I'm quite happy to pootle along at 50mph, but it very much depends on the type of road and the quantity of traffic using the road. If it's a trunk road I'll stick to the speed limit but if I'm driving on a secondary A road, or more likely a B road in my part of the world, I don't see a problem with driving a bit more slowly. If it's busy I'll keep up with the traffic or if it's a bit quieter and someone suddenly appears in my rear-view mirror, I'll allow them to overtake at the earliest opportunity. Even if I am driving at the speed limit, there's always someone who wants to go a bit quicker and I'd rather let them by - I just think that's good road manners.
That's certainly not good road manners and I made my feelings known about the "40 everywhere" brigage (your 38 is with the margin of error)!
I'm aware that not everyone will agree with my views but please do bear in mind that I inhabit part of the country that has no motorways and the nearest bit of 2-lane dual carriageaway is about 40 miles away. It gets very busy at rush hour and I often have to deal with traffic congestion in town but my local motoring issues are probably quite minor compared to those living further south.
I suppose it's all relative, really.