If you want to get discounted theatre tickets in London, you can either go to the Tkts booth in Leicester Square (expect queues) or try a site such as Lastminute.com which can offer discounts or packages for the theatre. The queue at the Tkts booth can be quite long so you can check availability on the Official London Theatre Tickets website. The other ticket booths you see dotted around London typically share the same inventory so there is little difference to whether you go to one or the other (the service charge may just differ).