Yet there's clearly still demand for it if Overclockers are charging ~£200 markup on some of their prebuilt PCs.
Computer building and overclocking are different things and require different skillsets.
It's a viable thing for OCUK to do because they've made a name for themselves doing it. Also, the people who are willing to pay for someone else to professionally overclock a system would probably go with someone like OCUK rather than a independent third party.