As Mushroom25 indicates, using the Trainline website is little short of madness. Third party websites can't provide tickets any cheaper than you'll find here
http://www.nationalrail.co.uk/
and often add on a booking fee, actually making them dearer.
Further, I use to run a railway station and frequently encountered tickets issued by The Trainline that weren't valid for travel at all! (e.g. they'd be issued using an offer only available on the services of Company A, for a train operated by Company B, with seat reservations on train that didn't even have numbered seats anyway!).
The price for 'walk up' fares (as they're known in the rail industry) is always the same irrespective of whether they're purchased in advance (either from a rail station or through the National Rail website) or on the day of travel (either from a station or, where the departure station is unmanned, on the train).
Cheaper 'advance' fares aren't available on all routes but, where they are available, they'll cost just the same at a rail station as they do online (unless you're silly enough to book through a third party website with booking fees).
Chris