Cotswold Woollen Weavers sounds like a very visit-worthy place:
"The smell of wool oil and the clack of the shuttles welcome visitors who tour our traditional 18th century woollen-mill set in the beautiful English Cotswolds"
British Coloured Sheep Breeders' Association is a very useful and interesting website to visit. They will be at Wonderwool...
An indie wool processing firm: Diamond Fibres
and I think this one is probably the best find of the day, and one I'll certainly be using: World of Wool (this links through to their Special Offers page) This is a website run by one of the largest commercial yarn companies in the UK, Europa Yarns Ltd. They say: "We do produce a selection of British yarns. We make aran, dk, lace and sock weight. All of which are on Shetland, BFL, Jacob and Black Welsh. All our british wool is sourced and bought through the wool auctions and is processed here in the UK."
LYS owners: "If you can order larger quantities than this we can send our wholsale catalogue". The prices are really quite reasonable, and they've got a fantastic range of yarns too.