A mandatory readback is MANDATORY, like it or not it has to be done, As a twr/apr controller I find that both US and UK crews are equally negligent in the readback area, US crews flying into my airport are mainly guilty of not reading back the type of service they are receiving (RAS/RCS), UK crews have problems with QNH, it's swings and roundabouts, If they miss a readback a couple of times, I tell them they are requires to readback in full the ???? they generally do, if not (as has happened a few times with Spanish crew), then they are told to hold position (if on the ground) or to route to the hold (if on the approach), it's remarkable how the understanding of the English language can change if they think a delay might result!!