Strangely, I remember them arriving. Nothing to do with aviation but we had an office in Southend I needed to visit, who always used the Airport Hotel, alongside the north side of the runway at the 05 end. Would have been spring 1981, when the BA fleet was purchased, that a new one would arrive from Cardiff in the late afternoons while I was staying there. It's a bit difficult to miss a taxying Viscount when you are within half a mile of it. They got about a dozen then, and a further half-dozen came in 1984, I believe having stood at the BA base at Cardiff for a few years.
BAF at the time had quite a few overnight freight runs from Southend to Europe, and the Viscounts very quickly mixed in with the Heralds for the several about 0100 departures, which of course woke us all up as each in turn spooled up for departure. They always seem to have been on easterlies.
BAF wasn't just a "handful" of Heralds, they had quite a substantial fleet (I see they had 15 in 1981 when the Viscounts came) but despite being a bit newer than the replacing Viscounts their manufacturer had gone out of business and they would be difficult to support. The Heralds had been acquired the same way, buying up blocks of a fleet being retired; they bought up the whole Malaysian Air Force fleet of about 9 in 1977. They probably found their leasing market, which is what much of their fleet did, was looking for something more of the Viscount size. They got some good longer term contracts, for overnight package runs and also from oil company support, particularly in North Africa. As usual in the era they had contracts outstationed in Aberdeen. They had not long got out of the cross-channel car operation, which was their origin; in 1981 there was still the last Carvair lying derelict at Southend in the weeds just outside my hotel room window.
BAF had a small group of scheduled services from Southend to nearby European points which they had let go of, to BIA, in 1979, but that was really just the commercial side as BAF continued to provide the aircraft, BIA did not bring in their own fleet.
Subsequently a onetime girlfriend (still long ago) had a father who had once been a Herald pilot with them for a few years; sounded a pretty straightforward day job as she said in the family BAF meant "Back At Five", likely a comparison to a previous position.
There's a detailed article about BAF here
https://www.southendtimeline.com/britishairferries.htm