You could do worse than Contour Fine Tooling, based in Stevenage, Hertfordshire. I've been using them for over ten years and the finish I get on plastics is second to none (optical quality). You cannot use diamonds to machine ferous metals though cos the diamond graphitises, but anything else machines beautifuly. Speak to them and they'll give you a solution.
Web site is www.contour-diamonds.com
Another company, in the UK, is Apex
Hope this helps!
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