Looking at the number of VOR's that have been retained I don't think that any place in the UK will be out of range of less than two of them if you are above the MSA.
Why would a PPL student need to know the MSA (which is an IFR term). If you mean they should now plan to fly high enough to always be in range of 2 VORs I am not sure that is practical (cannot go high enough due cloud or, especially in SE UK, controlled airspace).
Why would GPS result in people getting lost ? In terms of reliability is the ADF better ? ......... I think not !
A GPS, per se, should prevent pilots from getting lost.
BUT it should not be the
sole method of navigation and out of sight of the surface (VFR On Top) the back up system should, to my mind, be a completely different system.
And finaly why should a navigation equipment failure automatically result in a PAN call ?
Although the CAA encourages pilots not to faff around if lost and to seek help (Pan Call) earlier, rather than later, there are a lot of pilots who would rather request a 'Practice Pan' or 'Training Fix' from 121.5 than admit they are lost.
This isn't a major problem if in sight of the surface but, I would suggest, 'VFR On Top' with no Nav Aids is a completely different scenario and certainly merits a Pan call - even if only 'temporarily uncertain of position'.