BMA would have planned to use their Viscounts, of which they had a fleet of 10 (and nothing else) in summer 1969. Not so much based there as just fill-ins on the local leg of flights from the mainland (principally East Midlands and Luton) serving both islands, which they were already operating, where they did not have local traffic rights. I believe it's long been a sore point about mainland carriers picking up local passengers as fill-ins on any inter-island legs, and the old (London-based) UK licensing authorities seemed to go backwards and forwards over time on whether to permit this or not.
Intra was started in 1969 when British United decided they were never going to make any money on their various Channel Island routes, inherited from the old Jersey Airlines. Some of the management went along with a reorganisation into a reduced British United Island Airways, later BIA, while others left and formed Intra Airways. They were denied a hoped-for licence for Jersey to Guernsey schedules, so for a couple of years existed on just charters with one DC3. The service to Gloucester Staverton, and another to Cambridge, came their way when BMA pulled off these routes, which they had been operating with Viscounts. Normally a DC3 in summer, the would charter an Islander from their erstwhile competitor Aurigny in the off-season. Something of a come-down from BMA's Viscounts.
Intra is of course, after various name and management changes along the way, what we now know as Flybe.