.886 was MNO on the RAF VC10s as well, but MMO was higher, (.925 I think). To prevent the record being beaten until HOTOL came along, they got authorisation to cruise at 0.9.
The RAF Standards (Mk C1s) do have the tail fin tank - but they aren't actually standards. They had the standard fuselage but Super wings, gear and engines. So they are really shorter Supers with a freight door.