There was that wannabe-model Josie Cunningham (how she could become a model with a face like that I'll never know but that's a separate issue..) who got a boob job on the NHS by claiming that her flat-chestedness was causing emotional distress or some such like, so I'd guess that you can get the reverse done by claiming a similar thing.
99.9% of boobs are good boobs though, I don't see why one would want to alter them in any way assuming you're not trans or summin