Special Project pillars
A few trig pillars were built not for the Retriangulation but for other projects, and aren't shown on 1:50:000 scale OS mapping. Some are associated with one-off projects, such as:
British Leyland Works, Cumbria http://trigpointing.uk/trig/7511
Zeneca, Verdley House, Sussex http://trigpointing.uk/trig/7421 (uncertain whether this one was actually built by the OS)
Larger scale projects involving larger numbers of pillars, are Project Emily (dating from the Cold War) and the Great Glen Project.
One special project pillar which was incorporated into the OSGB network, is Shelton Bar http://trigpointing.uk/trig/5931, built for the 1986 Stoke on Trent Garden Festival. It was the last OSGB trig pillar to be built, in 1985, and was computed in 1987