Britain has retired its six L-1011 aerial refueling tankers as it introduces new A-330 MRTT tankers. The L-1011 tankers are still in good shape and have been purchased by a commercial operation (AGD Systems) that will use four of the L-1011s as commercial aerial tankers while the other two L-1011 will be converted to handle cargo operations (often for military customers.) Thus all six of these aerial tankers will continue to service military aircraft but now as a contractor rather than part of an air force.
This is nothing new, one of the first commercial aerial refueling services, Omega Air, uses two Boeing 707s (the civilian version of the KC-135) and a converted DC-10 for aerial refueling and delivers fuel at less than half of what it costs the U.S. Air Force (about $5 dollars a liter/$20 a gallon using KC-135s and KC-10s).
A growing number of countries besides Britain and the United States also outsource for some of their aerial refueling needs. They do this because the Americans, in particular, have a lot of experience with this sort of thing. For example the U.S. Navy has been using Omega since 2001, as have some foreign air forces. The navy keeps renewing the Omega contract each year, indicating satisfaction with the arrangement. The navy uses Omega a lot for training exercises or long distance movement of combat aircraft that would be a hassle to reschedule if the air force tankers were delayed because of air force refueling needs.
The U.S. Navy, which often depends on U.S. Air Force KC-135 aerial tankers to refuel its aircraft in combat zones has sometimes also found it more convenient to use a civilian firm for aerial refueling service in the United States. The air force controls all the large tankers (the navy can use some smaller aircraft, even fighters, for refueling in a pinch) and makes them available to the navy and other foreigners only when the air force has taken care of its own needs. Thus, non-air force users must sometimes wait. Omega Air allows the navy to avoid the wait by using commercial aerial tankers and many other nations are finding it useful to follow this example.