After my recent Edinburgh to Weymouth trip, I would seriously suggest that the cheapest way to do your journey could be to work out the route that it most likely to be delayed, and then count on getting half or all of your money back.
recenttraintimes will show you the delays for particular trains on recent dates. Then use any journey planner to find a theoretically quick journey with several changes. Chances are you’ll miss at least one of the connections and be delayed till the next train, an hour later.
Of course your personal finances will determine whether it’s better to be sure of a low price, or to have a chance of a low or zero price.
You can, of course, combine this with split ticketting.