With regards to the IAM advocating breaking speed limits, ANY breach of the law during test should mean a failure.
As Parksy says, the law is the law, whether we like it or not. There are no excuses. I am cdertainly not whiter than white, and would never claim to be (clean licence, yes) but unlike a great number of others to whom the law doesn't seem to apply, I try to behave.
Should you be unfortunate enough to be involved in a serious injury/fatal collision, there are many ways of finding out a vehicle's pre-collision speed, and I for one wouldn't like to have to live with the fact that it was my careless/stupid actions that were even partially the cause, even if I wasn't prosecuted. Where I live there are several mothers who take their children to school (yes, five minutes later, they're back), but the speeds that many of them drive around a housing estate road is almost unbelievable. It would be interesting to see what would happen if one knocked down a "little darling".
Cars are quicker and as technology develops, they get safer and safer, but the problem is always going to be the weak link - the nut holding the wheel (sorry, the driver). From studies, it has been shown that the average reaction time of a driver in the real world (not the stupid Highway Code laboratory test) is between one and two seconds, and some don't react at all (I've seen the aftermath of that a number of times). When travelling even at 60 MPH, in two seconds, a vehicle has travelled 26.82 metres. With a two second reaction time, that is nearly 27 metres before the driver has done anything. At 80 MPH, that becomes nearly 37 metres!
Finally, under certain conditions you can drive at speeds of up to 130 kph just the other side of the Channel. That's the law there. However, once you get to this side, the law says 60 MPH. If you travelled at 70 - 80 with the caravan, I believe that you would be prosecuted (if caught) for exceeding the speed limit for the class of vehicle, not Dangerous Driving.