Hello!
Nice to see somebody from Glasgow - I used to live there too.
The costs associated with studying in London can definitely be very off-putting. It was one of my concerns when I was deciding whether or not to apply to Imperial. Still, here's a few things to help you decide...
Travel to London from Glasgow isn't as horrendous as you may imagine. If you book well in advance, it is possible to get a one way train ticket for around 20 pounds, or 14 pounds if you invest in a railcard. You can get cheaper if you're willing to sit in a megabus.
Good accommodation in London is expensive: a single room here will cost you more than in Oxford, and will probably be smaller too. But there's a lot of variety too: you can choose to live in a shared room, or slightly further away from campus, and that will dramatically reduce the cost. Most people who have shared a room seem to have liked it, or never had any problems at least.
The biggest extra cost you'll face going to an English uni instead of a Scottish one is of course the tuition fees. I guess the biggest question is whether it is worth it. The Imperial maths course is considerably more fast-paced than the Glasgow and Edinburgh ones (which I imagine are the best unis for maths in Scotland), even compared to the three year fast track courses. Consequently, you will be taught a lot more maths. There are some really great lecturers here, and there is a much wider range of courses to choose from in third/fourth year. In terms of employment prospects/starting salary, Imperial seem to do better than Glasgow/Edinburgh too.
I guess if you took out the maximum loan, you'd be pretty comfortable living in London, with the £6000 from Imperial, ~£1000 grant and the ~£5000 loan from SAAS, and the tuition fee covered by SAAS. Of course you'd have to pay it back though... Many people get a part time job or a summer job to help cover costs and boost their CV -- you could look into that?
Hope that helped a little
Best of luck with your application!