Originally Posted by
corporate-wage-slave
You can also download the MVV MobileTicket App to buy your ticket online rather than trying to get the machine to swallow Euro notes. Many German cities have apps for tickets (Hamburg and Bremen spring to mind).
I agree with this advice, download the app and you can purchase your ticket while still in London. I've not tried the Android version of the app but on the iPhone the app store entry was in German so I wasn't sure whether the app would be available in English, good news was that the app is well translated and fairly simple to use (although not perfect).
The tickets you purchase in the app don't need to be activated immediately so if there's an issue with your flight and you don't arrive until the next day then this is still no issue.