I'm no master at this, but I have gained a few cheap tickets.1st class from Milton Keynes to Lake District about £27 each way. Cheaper than standard class at time of booking.
Milton Keynes to Brighton for £7.50
Look well before you travel. I go to national Rail Enquiries.
I usually look for routes that would compete with budget airlines, such as London-Edinburgh.
Flexibility helps too. The more flexible you can be about travel times and even travel day, the more chance you have of bagging a bargain. My £27 ticket to the lakes was the only train on that day several weeks before I travelled. They were at oddish times, but were about half the price of the next cheapest. they were some kind of promotional offer.
I usualy sweep through the time frame of my flexibilty to travel to find the cheapest.
Also, breaking down the train journey can make a huge difference. But not always.
I never get anal about it. There are too many variables to consider when you start breaking down journeys and seeing what you can come up with. I tend to take it if it's a good bargain or an essential journey, or leave it's not the bargain I was hoping for after about half an hour of fiddling about. I sometimes have a look on the off chance.
From the West Country to up north, I'd try via london and via Birmingham and maybe split from West Country to London or Brum, then split from London or Brum to you destination.
Also, a bit of a long shot if you're aiming to travel at a specific time, enlist in the rail companies e-mailing list. They send the occasional email when they have special offers on.