Further to this, all the train companies can sell tickets for each -other's services. This has a few advantages:
1) Virgin's site uses the trainline as its backend, and is very similar to the trainline in use, - in fact I believe your trainline login will work on the virgin site! By buying through them you save the credit card fee
2) If you take advantage of advance tickets, then I'd recommend the "national express east coast" booking system, which makes it really easy to find which trains have the cheapest tickets available. - When making seat reservations with them, you can also choose whether you want to face front or backwards (as well as the usual window/aisle/table options) which is a nice bonus. This site also doesnt charge you the credit card fee.