Not buying a return ticket would cost me double on my most-made journeys, where a return is only 10p more than a single!
The advise in The Mirror is over-simplistic. With pricing policy varying between a return fare being twice the price of a single, and 10p more than a single, there can be no common advise on all returns.
Melksham to London, walk up fares in three groups (off peak, super off peak, and any time) there should be 9 possible combinations (up O S or A, back O S or A in any combination). In practise, there are only six combo you would want (up S or A as there are no trains at O times!!, back O S or A). The Melksham Rail User Group web site at
http://www.mrug.org.uk/londonfares.html lists all six combos ...
three are best bought as two singles, three are best bought as returns!
As an aside (and I know I sound like a well worn record) I am still amazed that the "quick buy" ticket on the front of the
TVM▸ is the O (off peak) one that isn't the best value for a single train to London!