Well i came across one student/client who had received a GMAT waiver, but his was a pretty exceptional profile with a strong base in maths (as he had a UG in accounting & finance from LSE) and additionally he was an international level chess player, so that may be one reason LBS didn't feel the need to test him via the GMAT.
Also some other major reasons may have been that
1) He was applying really late in the admission process, around early July 2015. So he did not have the time to prepare for and write the GMAT
2) It might be that LBS would be in a rush to fill up its last few seats and hence could slide the GMAT requirement for students it deemed qualified
So yeah one formula for skipping the GMAT requirement in LBS maybe have a really great profile and then apply really late in the admission cycle
Though i'd rather prepare for the GMAT and play it safe.
Hope the answer helped.
You can check up
https://www.mim-essay.com/lbs-mim-review.html for a detailed review about the LBS MiM and also its application requirements.