Not fortunate enough to be considered centurion, however I use the platinum concierge for travel mostly as that is where I had the most success. My instructions to them are usually quite straight forward - desired airline carrier(s), cabin class and max budget for the trip details and leave the rest to them to come back - it gets tricky once you start introducing additional filters like transit/stopover, departure times etc...and at that point you might as well do it yourself.

In AU/NZ one could instruct and confirm the booking with authorisation via email [and the itinerary is sent when done]; in UK I found a return call is required (always) which I do find pointless, I would get potential itinerary options and then have to pick up a phone to the concierge team to proceed. (Mail boxes have a No Reply instruction).