Emergency situations involving London Control on 121.50 are covered in depth during groundschool studies and examined during Air Law and Communications written exams so unless I feel a particular student is more likely to get lost than is the norm I merely remind them about it's uses and demonstrate how effective they are by making a quick practice pan call to obtain a position fix and bearing for our home aerodrome at the end of a navigation lesson. However, if during a dual navigation exercise a student has become lost and following lost procedure remains lost I will invite them to contact London Control on 121.50 to make a practice pan call so that they might get first hand experience of their excellent service and hopefully learn a lesson (usually 'PPPPPP'!).
VFE.