"The L1011 was 'sold' to the RAF to make Thatcher's privatisation of BA work, to the tune of 60million quid. At the time the press announced they had been gifted, free, along with the spares - just as the VC10s had been earlier. Otherwise the RAF would have had their preferred second-hand DC10s, which if memory serves correctly were offered by Branniff (who had presumably gone bust by then??)"
It is not true to say that the Tristars were 'gifted', however, the basic airframes and engines were acquired by MoD at no cost to UK plc because UK plc already owned them (British Airways being a nationalised entity at that time). The MoD requirement was for 4 strategic tankers and the stated preference was for the DC10-CF30. The DC10 bidding consortium which, I believe, included British Aerospace, were offering some ex- World Airways aircraft (World Airways were a bit strapped for cash at that time). When the procurement recommendation was submitted for Prime Ministerial approval, 'She Who Must Be Obeyed' quickly spotted that the choice was between forking out scarce dollars to buy the DC10s (remember that UK plc was, itself, a bit boracic at the time) and effecting a paper transfer of the Tristars from BA to MoD and keeping the cash within the Treasury. From that point of view, a no brainer. Moreover, the Tristars had British engines so any money spent on future engine support would stay onshore. Hence, the RAF got 6 strategic tankers against a requirement for 4 (which was the original reason why 2 of the 6 did not get the full conversion to tanker/freighter config) and BA got some 'unwanted' assets off its books.
Alex W has pointed out BA's apocryphal faux pas of counting 4 engines-worth of costs against the Tristar in an evaluation against the 747. Whether that is true or not, BA certainly was a bit sharp in arranging the temporary retention of 2 of the Tristar-500s (G-BFCA and G-BFCE) to serve their South American routes until these frames were required to enter the Tanker conversion programme and, thereafter, leasing 2 aircraft (1235 and 1236) from Air Lanka. These 2 aircraft, G-BLUS and G-BLUT, had digital flight systems which had not previously been certified by the UK CAA and it must have cost BA a bob or two to get them on to the UK Register so perhaps there is some truth in the tale.