I've got friends in both

I've got friends in both programmes and both are basically not worth their money - you are paying for the brand name.

That said, I'd still go with Imperial if these are your only two choices, because it's at least relevant.
Grades don't matter anyway, since you are interviewing for full time before you have received the majority of your grades and I have not heard of anyone who needed sth. higher than a pass in their contracts.

And why the MSc Finance at the LSE is so highly sought-after by applicants is beyond my recognition.