Britannia 707s did operate from Luton. Certainly did some ABC (advanced booking charters) to the US including I think Los Angeles which required a tech stop outbound, although these were also (more usually?) operated from Stansted. Also ad hoc charters- I certainly remember one of them operating to Kuala Lumpur (direct so probably empty given take off weight restrictions) and a regular Tenerife ex LTN, probably on a Saturday in 71/72.
Remember on cross-windy day when the 707 went around although a MON 720B on approach ahead of it landed.