Potnoodle - the best London option is
Condor Cycles in Greys Inn road. Very good, but you pay for what you get.
Other slightly cheaper option but arguably as good is mail order from Spa Cycles of Harrogate
I got mine from Spa Cycles and 3,000 miles of SE London commuting and 4 weeks of loaded touring and they have not so much as requied the attention of a spoke key yet.
I'd also be inclined to get a pair, otherwise you get a nice back wheel and a crap front one, so you just move the problem from one end of the bike to the other.
Get the builder to add in a few spare spokes to the package - I have 6 and hopefully will never need them