Ticket to Ride: London plays similarly to Ticket to Ride: New York, which is a scaled-down version of the TTR base game. Here's an overview of the gameplay:
Each player starts with a supply of 17 double-decker buses, two transportation cards in hand, and one or two destination tickets that show locations in London. On a turn, you either draw two transportation cards from the deck or the display of five face-up cards (or you take one face-up bus, which counts as all six colors in the game); or you claim a route on the board by discarding cards that match the color of the route being claimed (with any set of cards allowing you to claim a gray route); or you draw two destination tickets and keep at least one of them.
Players take turns until someone has no more than two buses in their supply, then each player takes one final turn, including the player who triggered the end of the game. Players then sum their points, scoring points for (1) the routes that they've claimed during the game, (2) the destination tickets that they've completed (by connecting the two locations on a ticket by a continuous line of their buses), and (3) the districts that they've connected. (A district consists of 2-4 locations, and you score 1-5 points for a district if you link all of its locations to one another with your buses.) You lose points for any uncompleted destination tickets, then whoever has the high score wins!
The press release announcing this title includes this quote from Moon: "Ticket to Ride: London builds on the excitement of Ticket to Ride: New York. It's slightly bigger, but it feels more wide open, and the new Bonus balances the board and the tickets. Ticket to Ride: New York had taxis, so naturally Ticket to Ride: London has double-decker buses. Can you guess what the piece will be in the next city game?" In case you had any doubt, Ticket to Ride will travel elsewhere in 2020. My money is on gondolas...
Updated image from the publisher on April 3, 2019 here.