Well, I'm not sure I agree.
Surely the flight to the examiner is not going to be a challenging x-country, but most likely a short positioning flight via a very familiar route to and from a very familiar airfield. And by the time a candidate is ready for a PPL exam, the outcome of the exam (a pass) should already be 99% sure. In that respect, the risks associated with it are minimal. Far less than the QXC, for instance.
But at the same time it may have enormous psychological benefits. Before and during the initial positioning flight you have already done a pre-flight, runup and flown the aircraft. It's working, and you've setup the radios and such exactly the way you want it. Nobody has touched the aircraft after you landed. You then collect the examiner, and can do the supervised preflight in the knowledge that the aircraft is OK. In fact, you'll probably have far more confidence and mental capacity to treat the examiner as a passenger than if this is your first flight in this aircraft today, and you have no idea in what state the previous student left it.
All that may continue through the rest of the flight. So I think the psychological advantage of just having done a solo flight (a short x-country even) to pick up the examiner may have tremendous benefits.
Obviously as the instructor signing out the student, I'd insist on receiving a call once the exam is over, and making sure the candidate had at least half an hour to an hour rest before the flight home. Just so that the adrenaline could wear off. And in the very unlikely event that the candidate failed, you can still sign him out to fly home on a student licence. (Which needs to be done in case of a pass too, because by then the licence will not have arrived back from the CAA yet.) Or, if he's not up to it, drive over and fly the plane yourself while the student drives your car back.