All available train fares are listed when you enter your journey details on the National Rail website:
http://www.nationalrail.co.uk/
The firms which claim to get you cheaper tickets are simply offering the same advance purchase tickets which you can get directly from the train operating companies. (You sometimes have to pay a booking fee, so you actually pay more, not less!). Additionally, the independent companies selling train tickets make quite a few errors. (I used to run a rail station and I got fed up of having to explain to passengers that the tickets they'd bought through agencies weren't valid for travel on any train).
Stick to using the National Rail website but remember the usual rules for finding cheap fares:
1. Book as far ahead as possible. (Ticket allocations are usually put on sale 12 weeks prior to the date of travel).
2. If possible, avoid peak days and times. (For example, you'll hardly ever find any bargains travelling during the morning or evening rush hours. Travel at, say, 11.00a.m. on a Tuesday often provides the best chance of a bargain).
Chris