I have been working this summer, but at the moment I would only be able to do 15/hrs a week because of just being on the student visa. I am not yet on FLR.
Under the student rules, you can work up to 20 hours a week during term-time, and without a limit during vacations (that's during specific official vacations as defined by your college and, sometimes, by your specific course).
Unfortunately though, you cannot rely on your UK earnings in any immigration application to show you have sufficient funds, unless it's earnings from a sandwich course placement, or work guaranteed in advance by your university, or your the earnings of your dependant.
Hope it works out for you.