Party lines (or Shared Service) were just a legacy of a growing demand for telephones. They were used to reduce costs on line plant and later exchange equipment and until the technology was developed to have separate lines.
In the early 1970s the 1+1 Carrier System (WB 900) was introduced. This allowed two customers to share a line in complete secrecy from one another. The Carrier system used a higher frequency for one party so they could not hear each other and each phone only rang if their line was called.
System X and System Y digital telephone exchanges were introduced in the UK during the late 1970s and early 1980s which did not require Shared Service.
Privatisation plans for BT were not announced until 1982 so party lines were nothing to do with politics and political parties but all to do with technology development and capacity.
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