Probably "Sunny Jim" Callaghan off somewhere, it wasn't the Queen departing on a State Visit. Log says I also saw XV103 at Heathrow in April '78. The rest of the C.1s were all seen at Brize on several visits, other than XV101 which displayed at Biggin Hill in '76.
Just remembered the last time I saw any VC-10 fly was at the Biggin Air Fair in 2010 - great to see one flying at the event again but it landed too and stayed the night. I saw it the next afternoon heading west past Kenley at a couple of thousand feet while I was driving back from a Rapide flight at Shoreham! That was also XV101 apparently.