RLP
The Vanguards operated for a couple of years, summer only. I think it was once a day on Thursdays & Saturdays. They were listed on the BA timetables. But, as you say, they were few & far between. Jersey had a regular, & long lasted service with them from Heathrow two or three daily year round. I flew in them twice- GAPEI both times !
As you say, the cargo flights with Merchantmen lasted a lot longer &, as far as Guernsey was concerned, much more frequently than the pax service.