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Arup Sen (on here as @arup) is very good, if you can make it to deepest darkest SE London, and he's happy to use parts supplied by a customer. Condor Cycles will also do so, but IME they're a bit rubbish. I left some rims and hubs to be built up by them three weeks ago, having been assured categorically that they would be ready yesterday. They weren't, and they weren't ready today either. They didn't even have the spokes to do the build yet. I'm also somewhat miffed that when I dropped the hubs off they had track nuts on them, and now they don't (I retrieved the hubs and rims today so I can build them up myself).
tl;dr - @arup is good, Condor are a bit crap, but both will do it
Whereabouts?