Transport for London (TfL) has a Single Fare Finder which allows you to determine the cost of a contactless/Oyster pay-as-you-go journey between any two stations on the London Underground, Docklands Light Railway, and National Rail stations in Greater London or served by London Overground or TfL Rail. Each journey will return a Peak fare and an Off Peak fare.
For journeys where multiple possible routes exist between the two stations, different fares may be available depending on the route taken (typically when a journey can be made whilst avoiding Zone 1). These are shown in addition to the default fare under Alternative Fares, with a note such as "changing at Vauxhall" to indicate where the fare is applicable. However, since the default route does not have any annotations, how do I determine what the default route should be when alternative fares are available?
For example, if I ask for the fare between Surbiton and Watford Junction, I get five possible fares: 3 variations via Zone 1 (all the same price, but via different stations), one avoiding Zones 1 and 2, and the default fare. However, if I use the Journey Planner for that journey today (27/10/18), all the results route me via Zone 1 - the default fare does not show up as an available option.