You might try Hatton Garden, which is London's jewelry district. It caters more to diamond and bridal jewelry (as do most of these sorts of districts) but there are many shops that carry pre-owned watches - though on my last walk through there was a massive Rolex bias. You can find pretty much any kind of basic GMT, Sub or Datejust in windows there. Many BLNRs were out. Haven't seen a whole lot of the haute horology stuff, but that doesn't mean it can't be found.
I guess it must be as Tried and True says of 47th. You've got to learn the lay of the land and get familiar with the goings on before you'll be at ease finding what you're looking for.
Agreed. As I feel like I just woke up in a dumpster after a 5-day horological bender, I'm not in the market for anything this trip, so no worries there. But it's always interesting to look at eye candy of one sort of another. I enjoy wandering around the stores specializing in scientific instrument antiques in the area north of Covent Garden, for example. Thanks for the tip!