if you're close to London, the fares are nuts. It's off-peak £24 from here (£16 with a rail card) and that's very restricted hours, making it almost useless for day trips - it wasn't so bad when there weren't evening restrictions, but now there are, so you can't leave til after 10, and by the time you've waited for the first train after 10 and got down there, it's close to 11.30, and you have to be back to get the train by 4.15 at the latest to come home, and it's barely time to do anything - and no room for delays!
If you go earlier, it's close to twice as much.
And yet, for those prices, you can sometimes get advanced fares all the way to Scotland! Or even not specially advance fares, but going in a different direction, and you go much further. In fact, you can go THROUGH London, en route to Cornwall, sometimes for cheaper than a ticket that finishes in London! (But restrictions on the ticket machines don't let you use it for just a trip to London)
You can get from London to Birmingham for about £5 off peak.
From where I am to Birmingham (which is actually a bit closer), it's £30.
Fares seem to have nothing to do with distance and cost of the journey at all - it's all about who they can charge most to to make the most profit.