Use viginfarefinder if your going via Preston I know from Glasgow it's about a third of the price going to Preston than Manchester direct. Over the years I've found if you break the trip up as suggested you get it much cheaper eg Glasgow to Carlisle then Carlisle to Preston/Manchester and just sit in the unreserved coach from Glasgow. Never had my ticket checked on Virgin from Preston to Glasgow either but get a ticket as Sod's law day I don't get one is the day they check it!! Check out megabus as well for midweek games they do a cheap train to Preston also