Yep - I've used a mill. Initially it was The Natural Fibre Co under its previous owner. I saved up my 25kgs of Heb fleece and had one lot of knitting yarn done, then another time I had a lovely Hebridean blanket woven, and joined a share scheme to have some Shetland spun and some woven into blankets. I also had a batch of Jacob sorted into colours and spun into three colours of yarn - no, four as one was all three plied together. That was years ago and I still use the Heb blanket on our bed, over the duvet, to keep us alive through the winter.
A couple of years ago I had two batches spun at Halifax Mill near Goole. I took the two giant sacks down as I was passing on the A1 and the guys were so nice. They gave me a cuppa and showed me round the plant. One batch was all first shear to which they added 15% black alpaca, to make knitting yarn. The other batch was coarser stuff which they wove into a stronger yarn for rug warp - which I will be weaving myself. Unfortunately I have been too ill since then to drive down with another batch, but maybe next year. I know I could send it, but I really enjoyed my visit so would like to see how the mill is progressing.