If nobody has mentioned it yet, Helmshore Textile Museums in Lancashire. There's a pre-industrial wool fulling mill and a more recent cotton yarn mill, both working, the former water-powered and the latter now electric-powered. You can watch the mule passing back and forth winding several kilometers of yarn with each pass and you can imagine the children crawling under the machine collecting the fluff. There's a very good audio-visual display in the basement, which explains the importance of weavers in pre-industrial Britain and gives you an appreciation of why the Luddites smashed up the machines they saw taking away their jobs.