How does one take possession of London theatre tickets purchased in advance from the U.S.?

Typically tickets for overseas customers are held at the theater box office for collection on the day of the performance.
Before you give the site your credit card details the pickup process will be fully explained.

I generally buy from the agent that operates the official box office for the theatre. For Dr Faustus this is ATG
http://www.atgtickets.com/venues/duke-of-yorks

I've bought from ATG before. When you buy your tickets there will be an option to collect the tickets at the theatre box office. You will simply go to the theatre about half an hour before the show starts to collect the tickets. Sometimes they will ask to see the credit card you used.

You can go to http://www.theatremonkey.com to find reviews of seats and also other agents that sell tickets.

Thanks for this info everyone. Let me clarify a question I had though and add to it: I know seat location and date affect price, but do different ticket sellers have different prices from each other for equivalent seats on the same date? If I see, say ten seats available from one ticket seller, might I find more or fewer from another? Is it worth my while to shop around online or if I find a ticket for an acceptable seat for an acceptable price should I assume no other seller will be cheaper? Thanks again! Ann.

I have bought tickets in advance (from home in the States) for theatre in London on several occasions, and for several different theatres, but only directly from the theatre's website, over the phone with the theatre, or from the seller that the theatre's website directs me to, never from a reseller. As far as I know, the only authorised ticket reseller is the TKTS booth in Leicester Square

Every time, we have picked our tickets up at the theatre on the day of performance (what we call "will call" in the States) and we have never had a problem. I think the only time we had to show ID and the credit card we used was for Cumberbatch's Hamlet last year.

I hope you are able to get tickets for the production(s) you want, and that you have an enjoyable trip!

I had to claim my ticket in person at the Bariban will call window last October for Hamlet. I had printed out a copy of my email receipt just in case, but it wasn't necessary. I just showed ID and the ticket was in an envelope with my name on it. No problem.

As far as I know, the only authorised ticket reseller is the TKTS
booth in Leicester Square

Not so. A lot of people purchase advance tickets via Ticketmaster UK. They are as authorized as one can get in the ticketing world.

Different ticket sellers apply different fees. Usually the agent that runs the theatre's box office has the lowest fees and the best choice of seats. Occasionally, you can find discounted tickets online, but this tends to be more likely with less popular shows.

For example, as I noted above, ATG runs the box office for Duke of York. ATG has a 3.50 GBP fee per transaction for Doctor Faustus. On the other hand, for this performance, Ticketmaster charges a fee of between 4.90 and 11.90 GBP per ticket PLUS 3.20 GBP per booking. On both sites, the seats themselves are the same price. Only the fees are different. There are many authorize resellers. Agents that are members of S.T.A.R are reputable.

Theatremonkey is an excellent site for researching the various ticket sellers and seating choices.

http://www.theatremonkey.com/DUKEOFYORKbooking.htm

I cheerfully stand corrected, but just noticed I left a key phrase out of my sentence: in the Leicester Square area. What I was trying to say (and didn't do well, my apologies) was to warn people off the many dodgy ticket resale shops in the Leicester Square area, in favor of the TKTS booth.

Mea culpa!

Great information. Thanks for all the detail! Ann.