I tried the new BA system on T5 two months ago..... fantastic....
I didnt even notice I was a non rev pax.

I listed on the flight to CPT at their web site about a week before my intented flight date.
I showed up at the airport about 2,5 hours before...
Right at the airport you will see plenty of easy check-in machines, which will check you in. It will ask you some questions and then it will print out a standby boarding pass, with your priority (the system knows if the employee is a one world employee or not, and assing different priorities for each case), and record locator reference.

Then, we went to the room for clearing standby's... It is exactly at the left hand of the check-in hall... We asked, and an agent told us.
As we took a look, we saw that there are many seats that people use to rest before the their flights get resolved. Also there are a couple of computer to list yourself if you have arrived at the airport without a listing. Remember that there is no other way of listing but online, and also there it will be no one at the room to help people and give loads... There are only some families and people waiting for a seat.
It is also clearly explained that it is unnecesary to wait there long time before, as the flight information is only exposed just one hour before departure. So we went to have a sandwich right next to the room, at the Cafe Nero.

If you have listed properly and put your mobile number at the reservation, they text you a message telling your status onboard, but I will explain it later.

Right before an hour for the departure, we approached at the room, and we sat. As we only had hand luggage, we had it on us, but if someone has to check any suitcase, you have to tag it at any bag drop even before you know if you are accepted on the flight, and then keep the luggage with you waiting for the final result.
at the room there where many LCD tv on a wall, and you have to look for your flight. Then you will see your record locator number before the title of your flight, and it shows an status, for example: WAITING SEAT... or ACCEPTED, or NOT ACCEPTED BUT RESILTED ON THE NEXT FLIGHT. there are some pannels where BA explains what to do at any status, so do not panic...
As they started accepting people, passengers were leaving the room and then they went the famous BLUE OR RED DESK, which are just at the other side of the corridor when you get out of the room. Two meters of distance!!
So lets just see the possibilities:
1.- you have been accepted: the screen will say ACCEPTED GO TO BLUE DESK... so you cross the corridor, and there are like two check-in possitions with agents that will check your ZED/ID ticket, and will handle you boarding passes with seats. Then you ask where to leave your suitcases that you tagged before.
2.- NOT ACCEPTED, RELISTED ON THE NEXT FLIGHT: BA automaticaly relist you on the next flight (if any) for the same destination... you dont even have to do anything... just wait for the next flight...
3.- NOT ACCEPTED, GO TO THE RED DEST: I suppose it means that you have not been accepted, and they want to ask you something... like if you want to be relisted for other destination, or if you want to split your party, or something like that.

One second after the screen said to us that we were accepted, my Spanish mobile phone received an SMS, saying: BA ACCEPTED ON THE FLIGHT, PLEASE CONTACT THE BLUE DESK OR ANY FAST BAG DROP... I didnt know that it even worked for International mobile numbers, but it did work.

Then you have to run to your gate, because it is usually far, and you have to first go to check your passport to any of PASSPORT AND DOCUMENT CONTROL AGENT at the check-in areas.................., then go through Security........... then we took a train to our gate, and then board....

BA was sooo nice that gave us premium economy, which is a quieter cabin with a little wider seats, and nice catering.... lovely experience if you see what I paid for the zed...

My partner was amazed. So was I.

I hope it helps, and sorry about my English.

J.