I used to run a railway station and my advice is that you should NEVER buy from the Trainline site (or from any other 'third party'). Such companies have a nasty habit of making errors, resulting in travellers being issued with tickets which aren't valid on any train. (I've seen loads of such tickets).
Check the available train fares on the National Rail website:
http://www.nationalrail.co.uk/
Then click on the links (or go direct to the relevant train operating company's website) to purchase your tickets.
Chris