Firstly, the ONLY rail website you should be looking at is the National Rail one. Site's such as the dreaded Trainline.com can't offer tickets any cheaper and often get things wrong. (When I ran a railway station I got fed up of explaining to customers that the tickets that they'd bought from Trainline weren't valid because, for example, they'd been sold them using an offer from Company A but booked onto a train operated by Company B, with compulsory seat reservations on a train that didn't even have seat numbering):
http://www.nationalrail.co.uk/
However you won't be able to find tickets offered on the National Rail website for August, as they're not on sale yet.
The regular fare is £43.90 and it's unlikely that any cheaper 'Advance' fares will be offered on that route.
Typical journey times are around 3½ hours. (It's possible to do it in about 3 hours but that involves an extra change; it's all explained on the National Rail website - just hover your cursor over the 'Changes' number to see what's involved).
If economy is important, National Express can get you there by coach (5¼ hours) for around £26.