Hello AspirantSC,
I'm not sure who is telling you that an 8.31 is too low a GPA for the MIM at LBS. Your GPA is competitive for the program, and although South Asian students and some other demographics might have more competitive GPAs than you, your GPA is more than sufficient. You should also remember that it is only one aspect of your overall application. Scoring very well on the GMAT is a way you can easily boost your academic profile, and if you match that with a stellar application, then the LBS MIM is definitely within reach for you.
Don't let people dissuade you otherwise,
Hadi