I should add that - while it's always the easiest - contactless payment is usually but not always the cheapest method of payment for train fares, especially on routes beyond zones 6.

Sometimes a specific National Rail ticket is cheaper. This can include if you are making a day return, travelling at certain times (peak vs off-peak), have a railcard, etc. There are too many permutations to list out here but if you are interested in a specific route you can always check the contactless 'single fare' price on TfL and compare that to the tickets available on National Rail.