Just noticed that TfL has launched a new facility on the fares section
of their website - a "Single fare finder".

http://www.tfl.gov.uk/tfl/fares-tickets/fare-finder/

You input your start and finish station - the fares finder is replete
with a javascript auto-complete function - then select adult or child
and and click on "Show fares".

The applicable fares are then listed - the far cheaper Oyster fares
comes first (both the peak and off peak fares are listed if there's a
difference), followed by the pricey cash/printed ticket fare.

It's basically just a friendly front end to a simple database that
lists which stations are in what zones.

National Rail fares are only listed on routes where there is
interavailable NR/LU ticketing - one needs to go to the National Rail
Journey Planner for anything else.

This basically seems to be a bit of smart marketing to demonstrate to
those who don't wish to/don't know how to/ can't be bothered to consult
the fares tables how much cheaper the Oyster fare is.