Shadow cabinets are a much older phenomenon in British politics than the 1980s and certainly date back at least to the early 20th century.
It was gradually institutionalised from the 1920s, and in the Labour party - from the 1950s. Traditionally, the Conservatives were more informal and left the appointment of the shadow ministers to their party leader, whereas Labour established a formal procedure for electing them from its MPs and
the leader would have only the right to divide the "shadow ministries" among them. Of course, when he won an election and became Prime Miniter - like Tony Blair - he would have so much power, that he would be able to assign them other ministries or ignore them altogether, but that could count as political disloyalty and damage him in the long run.