I'm pretty sure this has been mentioned before but with so many posts on here I am going to mention it again
If you are a PDI and unsure as to where to do the briefing just ask yourself
'is this a driving subject or a maneuvering subject? if it's a driving subject then give the briefing at the test centre, if not give a quick Q & A to establish prior knowledge and add the usual 'ok, lets drive out of here and I'll direct you to a place to carry out the manoeuvre, on the way I'll help you to correct any other faults'
Then do the briefing after arrival
Hope this helps