I have no issue with vehicles that can weigh over 40 tonnes driving at 40MPH on single carriageways, The amount of kinetic energy that they possess is incredibly high and you really don't want one to lose control at 56MPH (max speed).

Also, when they travel at 40MPH, they are considerably easier to overtake than when travelling faster.

Also, please don't forget that these vehicles are also large adverts for the companies that they have written on the side. Tesco/Sainsbury's etc don't really want to creat an image of poor or illegal driving.

If those same HGV's were travelling at 55 mph this would be a thread from someone complaining about speeding, doesn't matter what you do people will find something to complain about.

No speedo watching needed, nearly every truck has cruise control and all modern trucks have calibrated speedometers.

Just like to point out here that i don't travel at 40 mph i tend to run at a genuine 50ish, travelling at 40 causes the sort of frustration expressed by the OP, understandable too i drive a car when not at work.

40 mph driving does result in drivers overtaking in unsafe conditions, travel at 50 and a car will likely be showing 54 or 55 mph on their speedo, some still overtake but it tends to be the competent ones, better for all IMO though not right according to the law.