If the offers provided by buses do not suit your needs, trains and carpooling could represent a good alternative for this distance.
When the journey is so long, the bus takes much longer than other transportation. You can have a look at flights to Southampton: by booking in advance you can find very cheap tickets!
What time are the buses to Southampton?
The earliest time you can leave from Glasgow is at 11:03, while the last bus is at 22:03. The journey takes on average 12 hours and 56 minutes but it can also last 12 hours and 5 minutes with the fastest connections.
Which bus lines can take you from Glasgow to Southampton?
There are many companies cooperating with virail, among which also National Express with buses to Southampton.