WRTo the RAF Tristars. The RAF got bigger tanker/transport aircraft to fulfil the airbridge requirement to the Falklands which it needed in a hurry. It so happened that the then government owned BA had surplus aircraft - 6 Tristar 500s. The only reason it got them is because they were cheap and available. If a full procurement process had been carried out, it proably wouldn't have got the Tristars. But on the other hand, they would have only just been coming into service now!
All of the RAF Tristar guys who flew the KC10 on exchange agree that it is a far more capable and reliable aircraft.
Speaking as someone who has had his various scentences in HMP Mount Pleasant extended by 5 days due to Tristar unserviceablities!