I realise that this been discussed at length and is neatly summarised by Airprox above.
My daughter broke all the rules recently. She was flying to Nairobi in economy, on an award ticket that I booked using my Avios. She is a mere Silver card holder (thanks to my GGL largesse). She did not OLCI but was upgraded to WT+ at the airport check-in desk (she was rejected by a machine). My only thought is that she was a single traveller.