If it happened in a week, you should take the bike back to the shop you bought the bike from and get them to give you a new wheel (or at least fix the one you have) for free [generally, they should have you take the bike in for a service after ~30 days / 50 miles, whichever comes first, for free]. Any competent bike shop should be able to replace a few broken spokes or adjust a wheel.
Good wheel builders are rare and expensive, and you also have to make sure the quality of wheel is worth being rebuilt if you're going to pay for their services (if its a cheap wheel or there has been damage, they may not be willing to reuse parts or will charge more than just getting a new wheel). Going to a good wheel builder is for people who need/want exacting properties of their wheel -- strength for touring, extra heavy people, etc. in many cases.
That being said, I don't know any in London (and I'm not sure its within SE's format to ask that question).