Spotting a Heuer is rarely guaranteed in London unfortunately.

Some places worth trying though.

I usually arrive in London at Waterloo Station and walk from there, so it's quite natural I check out Austin Kaye on the Strand. They'll have more TAG-Heuers than Heuers - sometimes none at all, but I've seen some nice ones there. They've just sold a Carrera 2447, but watches are often on display before they make the website, so worth a peek.

http://www.austinkaye.co.uk/

From there, you can make your way west to the Bond Streets. Both Old and New have lots of watch dealerships, mainly at the higher end but the roads and arcades off of each have probably the highest concentration of second-hand watch dealers in London.

Grays Antiques Market, off South Molton Lane near New Bond St, has a number of watch dealers, you might get lucky and find a vintage Heuer, but more likely you'll see a lot of Rolexes and a few TAG-Heuers. That's how it is in most of the London dealers really.

http://www.graysantiques.com/

Burlington Arcade, off of Regent Street, is worth a look, if only to marvel at the ambitious prices! Again, mainly Rolex, but the Somlo shop at the end furthest from Regent St is authorised by Omega itself as a vintage watch seller.

http://www.omegavintage.com/omega-vintage

I normally round off by buying some nice wagashi, or Japanese sweets and desserts, in Minamoto Kitchoan back on Regent St.

http://www.kitchoan.com/

Not typically London perhaps, but the packaging is great and they make nice presents if you've been asked to bring things back from your trip to London :) I stumbled across their NYC branch by accident on my first trip there too.

Anyway, have a great time and let us know how you get on!