the QXC is a navigation excercise, so it should be done using DR, and this has to be the main tool in nav training. my point was that as the a/c approached the class A, the GPS would be bleeping or flashing MSG, or both. if the student knew how to operate the basics of the GPS, it should have alerted him to the impending infringement.
agree that thruxton/sywell is a long DR leg, and once past didcot/oxford, there are not many good ground features until you get to MK.