theatre ticket resale for London
 

I'm trying to find tickets being resold for a couple of sold-out shows in London. In the United States, I'm used to looking on StubHub and other such sites, but there seem to be no or very few tickets available on StubHub for London shows. My questions:
1. Are there other sites that are more popular for ticket resale?
2. When I arrive in town, are there either companies or perhaps particular hotel concierge desks that might be good for getting tickets?

Thanks much in advance.

Any hotel concierge should be able to get you tickets for whatever show you want - but the price may be enormous.

Hopefully someone local can send you to a local resale site in London.

have you looked on the ticketmaster uk site or
lastminute.com uk

there are ticket brokers, ask at your hotel.

I haven't been there in a while, but when I was, there were shops around Leicester Square doing that, like the Leicester Square Box Office (this is not the nonprofit TKTS booth).

Stubhub is used there, you can try this place https://www.gumtree.com/tickets/london/sold+out

here are some agencies and tips http://www.theatremonkey.com/soldouttickets.htm

Thanks to all -- these are all very useful resources.