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Essentially the NAIRU (the non-accelerating inflation rate of unemployment) is driven by the relative bargaining power of agents in the labour market. 

Unemployment in the UK continues to fall although under-employment remains high. Real wages are stagnant or falling for millions of people in work. Monopsony power in the labour market is growing. Barely a day goes by without news articles on employers such as Amazon, Sports Direct, Uber, Deliveroo and numerous contract cleaning and security businesses where the negotiating power of employees is close to non-existent - the expanding precariat is an issue of economic and social concern to many

Trade union membership has fallen from a high of 13.2 million in 1979 to 6.2 million in 2015-2016