Where do you get prices like those? Or is it just for very select routes?
It is heavily dependent on routes but there's a few general tips to save loads of money:
Stay with the same TOC, both the journeys listed are direct journeys. TOCs tend to offer big discounts on their own tickets but if it's a combined route its harder to get cheap tickets. For example Coventry to Nottingham (LM/VT then XC) never has dicounted tickets.
Book in advance. Tickest become availble 12 weeks in advance so start looking early.
Get a railcard, if you can it'll save you 33% off all journeys.
Additionally if you're looking to save even more money you can try split ticketing but that takes a bit of effort.
The complexity of the ticketing system means there are some really good deals out there. The given example of Coventry-Euston for £4.95 and a journey time of 1 hour is a brilliant deal. Nice relaxing journey and it's impossible to do it quicker or cheaper. Incredible really. However, if you turn up on the day you'll get shafted as the same journey is about £30 normally.