Hi
Honestly it will be more expensive during your first 6 months before you receive your BA/BMI concessions. I commuted from Glasgow after probabtion, but know people who moved straight back after training. They just had to book flights as soon as they received their roster - and it worked out quite dear, but they were confirmed seats - also you will have to give yourself plenty of time at the LHR/LGW side incase of any delays with the VS inbound flight which could end up costing you a small fortune trying to get home.
I think it is worth it though - it's 4 months realistically of doing this - 5 at a push - and you'll get used to it pretty quickly - if you are really brave you could always consider the bus?! National Express from Buchanan Street to Heathrow every night - if you get there early for flight - sneak into the crew sleeping chairs and rest up before check in. Don't be surprised at the lengths some crew go and travel to get to work - says a lot about the great job!!!
When your probation is up, and you get concessions - v important dont go sick at all in first 6 months as it'll knock getting your concessions back - not nice - use your concessions and buy 5 tickets a month - roughly £270, standby - but be ready for the performance it can mean trying to get to work form Glasgow standby especially as a Virgin Junior - good luck - you probably will be taking that bus every time! There are just a lot of BA staff who commute and take up loads of spare seats - then VS lot - but again - worth it - and you'll always have an airport pal to chat to!!
Hope it all works out well!!