I think that Whirlybird has summed up the situation very well. The problem is that the student thinks that because he has a JAR PPL he has all the knowledge and skills required to fly in the UK. We recently had a US trained student who performed perfectly well on the handling check but then hired the aircraft and flew straight through the Heathrow and Stansted zones enroute, in a straight line, to his destination.
If the new PPL appreciates that some of the money he has saved by learning in the US must be spent by learning the differences when flying in the UK, then fine. But how many schools in the US make that point clear? Preferably to the student before he starts?