So by buying two single tickets, you can do London - Glasgow - London for £27 on GNER (eg 11:00 on the Thursday, 16:00 on the Monday) or £35 on Virgin (eg 10:29 and 16:46, shorter journey times). The Virgin fare can be further reduced with valid railcards, but not the GNER fare.
For newcomers to the exciting world of modern train fares, the only difference between GNER's Standard Advance 1, 2, 3, and 4 tickets is the price, similarly with Virgin Value Advance A, B and C tickets.
Equally, 1st Advance and Value Advance 1st are first class tickets, identical to each other in all but digit/letter and price. This time, in shades of Spinal Tap, GNER's go up to 5.
Each come with a seat reservation and each is only valid for that particular train, so don't leave it too late to arrive at the station...