If you're returning on the Friday there should be some cheap advance tickets, If it's Saturday and there are no advance tickets then there are two options, the Crewe to Euston London Midland service but not too late as it will be very slow or a Cross Country to Brum and then the slow London Midland service which is only £3.95 with a railcard.
The Virgin trains are preferable as it's usually only about an hour and a half to Euston. It takes that to get to most places on the South Coast!