As a 10 year old spotter in very early 1975 (barely New Year) I recall watching a BA TriStar on a Dover departure over Purley, probably route proving I think. I'm not sure but think it may have been the first BA aircraft I saw in the full scheme, Tridents, VC-10s, etc were mostly still BEA/BOAC and possibly halfway in between...
Only TriStars I flew on were Delta Gatwick/Miami and return, about 20 years ago.