While that might be the case when you are doing split tickets or the like, 90% of non necessary expensive tickets are a result of people not knowing a few things about walk-up fares. Essentially you just need to know:
1. Travelling after about 9.30am will very often be very significantly cheaper, because you can then get an off peak return.
2. An off peak return is about the same price as a single - so never buy two singles when a return will do the job.
Now you go to National Rail Enquiries, and put in the days/times you want to travel, remembering to tick the 'return' box.
eg:
View attachment 278666
Now, press Go, and in most cases the results will come straight up:
View attachment 278667
There it is, straight away, the off peak return of which I spoke.
For most of the examples in this thread, following the above process will get you what you need, no special tricks. The prior knowledge required is not that advanced.