Just back from London so sharing some experiences.
It's London so of course one has to go to the theater. Decided to see Book of Mormon, which is about the most American play you can think of. Went to the theater the day of around noon, and they said sorry the box office opens around 4:30. So after giving them first crack went to the TKTS building. Note that this building is hard to find these days, it's not only surrounded by construction fencing but there's a Christmas market built in front of it. You might need to Google map it or look for the Odeon movie theater. Beware that when you come up out of the Leicester Square tube station there's a building that has a big marquee that says DISCOUNT TICKETS. This is not the droid you seek, keep walking.
Unlike the old days in NYC where there were actual tickets in cubbyholes sent over from the theater and you paid in cash, the TKTS operation today is totally modern and efficient. There's a TV monitor on the outside that scrolls through the available plays, you get to the window, do some back and forth, make your decision, and hand over a credit card. Not at all like buying counterfeit Vuitton or something. And even though it was day of, we ended up in the fifth row of the theater (which was full).
And in case you're wondering, no one was wearing a mask. Except my wife and I.