The Ordnance Survey has used a number of map projections over the years. Sometimes a record survives; in other cases it is necessary to infer the projection used from measurements on the maps themselves.
A late member, Brian Adams, found this a fascinating topic and wrote a number of articles over the years. After his death, these were collected together and published by the Society as Projections and Origins, which provides a wealth of information described in a manner accessible to the non-mathematician.
Brian had also written a summary, bringing together the salient points; and this is available here.