Just search 'anodising west sussex' on Google. I did the same for Portsmouth and there was a suprising number of little firms that do it. In my experience most of the smaller companies will do it almost as a favour as most of their work will be contract based for machining companies.
I had my motorbike frame, swingarm and back wheel done in satin black for £20 and recently my stripped down hope hub and RS1 crank parts done for £10.