As you clearly have internet access, it's much better to look for the tickets yourself. Not only will it mean you don't have to call the expensive phone number, you can also try different ticket combinations as indicated above to find the cheapest fare. All the staff on the phone have is an interface similar to the website, and they are unlikely to spend much time finding a cheaper fare than the one the system gives them first.
Use the website of a train operator e.g. Virgin. In theory you can use the site of any train company, but I'd suggest using the one that provides the main part of your journey as I recently was told some cheap tickets were unavailable, but when I tried the correct operator's site I could buy them.
Do not use thetrainline.com as they charge rip-off fees whether they post the tickets to you, or even if you collect them from a machine yourself, and they also charge credit/debit card fees. The train operators' own sites don't charge any extras.