The London Pass® is a digital sightseeing credits package that gives you access to 80+ attractions in the city. Just choose a duration for your credits package, then download the Go City app. You can use your order confirmation number to add your pass to your phone. Et voilà, it'll be right there on the app. (But don't worry, your pass won't activate until you visit your first attraction!).
Your credits package is valid for consecutive days, not 24-hour periods, so start early in the day to make the most of all the incredible London attractions at your fingertips.