What London tickets/passes should we purchase before travelling?

Transportation: Wait until you are in London to buy either Oyster cards or travelcards. This page on the London Toolkit site will help you decide which to buy. Theatre: If you want to be assured of getting tickets for a particular show on a particular day, book now on the Official London Theatre site. If you are willing to take your chances on getting day-of-show discount tickets, visit the TKTS ticket booth in Leicester Square. Click on the link on the Official London Theatre site for more information. Note: The TKTS booth does not offer tickets for all shows.

Thinking that we will go see Spamalot in London. My sister says it is awesome and we missed it in Chicago. So would it be wise to get tickets in advance for it? We arrive June 22nd and leave for Paris on June 25th. Dont think we would go in the 22nd while fighting jet lag.

It's completely hilarious. You will laugh till you cry. For best seats book in advance, though tickets will probably be available when you arrive. "Always look on the bright side of life..."

I just checked the TKTS site. Spamalot is on the list of shows for which TKTS sells tickets. And there is something new. For some shows TKTS now sells advance tickets (one or two days ahead). Again, Spamalot is included.