British union density reached near-compulsory levels in key industrial districts by 1895

Evidence

In Lancashire, Durham, and Northumberland the trade unions contained at least a tenth of the entire population, and half the adult male workers. It would be true to say that for the Lancashire cotton spinner or weaver, the miner in Durham or Northumberland, membership of a trade union was in practice compulsory. — Cedric J. Robinson, Black Marxism_ The Making of the Black…, 1212