Cost and time trade-offs
There is no separate off-peak rate to Heathrow, so the £15.50 fare does not drop at quieter times. Contactless and Oyster are charged exactly the same, and both are cheaper than a paper single bought from a ticket machine, so always tap in and tap out rather than buying a paper ticket. If you carry on using the same card around London that day, the Zone 1 to 6 daily cap of £16.30 means almost all of your later travel is effectively free once you have made the airport trip.