Ahhhh, those were the days...
If I remember correctly, there were two different services. One was provided by British Caledonian and the other by British Airways.
I got the chance to travel on the British Caledonian Airlink once as a combined Christmas and Birthday present from my Aunt. I think it must have been 1979 because I was about six years old. It was my first flight and we flew from Gatwick to Heathrow. My Aunt travelled with me, and my parents drove from Gatwick to pick us up at Heathrow. Needless to say, we arrived much sooner than they did!
I don't remember it being uncomfortable, but I do remember it was noisy and it was wonderful! The flight was over before I knew it, but I got to go on the flight deck, meet the crew and even got given a (very battered) safety card as a souvenir. (I still have it, and some photos somewhere)