From the Standard: No ticket, no excuse - train guards will show zero tolerance
South West Trains (my local railway in Putney and Barnes, with far better service than some of the lines in other regions) will no longer let passengers buy tickets on trains at reasonable prices.
This seems to be a policy aimed at causing needless delay and annoyance. At most there should be a somewhat higher price (not the outrageous "full fare" prices) for the convenience of paying onboard, rather than queueing (and perhaps missing the next train waiting in queue) to buy tickets. Happier passengers usually means more passengers.