The original wires were single non-insulated, hence the rows of insulators that used to be seen on poles. Some heritage railways still have them.
https://goo.gl/images/PgMh43
Then BT went to a “shotgun” type cable similar to what Virgin uses now (two insulated single wires). With the advent of dial-up internet & the requirement for more than one line at a property, the twin pair cable was introduced. Cables linked to poles now usually have 2 or 4 pairs.