The RAF operates 8 of them from Brize Norton as part of 99 Squadron. They were originally on loan for a certain amount of flying hours but it was quickly exceeded and now the RAF has flown the most flying hours with C17’s- so much now that Boeing has to ask them about any future problems that may occur as we have experienced almost every snag for over 100,000 flying hours!
And it takes a long time to refuel too- I did a couple in the Falkland Islands!