Even some longer distance journey's are cheaper to by a walkup ticket...!
Example - Taunton - Bristol is ^16.50 for an Off Peak Standard Period Return, A First Class Off Peak Period Return will set you back ^28.00
Now assuming you travel
XC▸ , It will cost ^15.00 Each way to upgrade! So purchasing a walkup ticket compared to the weekend first upgrades DOES sometimes make sense

Also breaking the Journey at Plymouth when travelling into Cornwall can make First Class Tickets considerably cheaper!
Another trap I discovered about 4 years ago when travelling to Yatton from Cornwall to watch out for is this one:
Liskeard - Yatton, Will set you back ^156.00 for a First Class Off Peak Return... However, Liskeard - Bristol Temple Meads will set you back ^142.00

For those not familiar with Yatton and Bristol...! They are on the Main Line from Taunton - Bristol Temple Meads... Yatton is 2 stops
before Bristol Temple Meads...!
So if you make that Journey, I would recommend buying the Bristol Ticket, Change on the Bristol Temple Meads or Cardiff Central at Taunton, and break the journey at Yatton
*edit* Wording Error