There are no classes in freshers' week but there may be a sort of registration session where you go along and find out information about the course. A lot of them do this rather than have an introductory course on the first day of term (so if FW is week 0, first week of term is week 1). If you miss this it isn't the end of the world - plenty of freshers are too hungover to attend anyway, but it's useful for easing your worries. If I remember correctly I had an introduction to the department more than the subject - so even though I was studying English Lit back then it covered everything in that school.
The events in the evening often run til 2am or later, with some events not starting til past midnight. I would check the line ups (which are being released beginning today and for the rest of the week on FB) and see if there's anything you want to see and then work out if you can crash in Glasgow or get home on a night bus or something. Don't spend the £45 if you're not gonna make the most of it, basically, since it's a lot of money. I didn't do freshers week in my first year cuz I lived in Irvine, but I'm a helper this year and I live in Glasgow now.
The events during the day around campus, such as all the flyering, stalls, freebies, freshers' address, union events before the evening, are all free and open even without a freshers' pass so even if you can't do the night time things, it could still be worth coming in for those things!