I'm all for mental health but giving people cosmetic surgery is not the way to go about it.
As for the limbs comparison, I do think it's quite different. Arguably even a prosthetic limb is more functional than larger breasts. And when it comes down to things like breasts, noses, etc. there's such a wide variety of shapes and sizes...whereas it really comes down to you either have an arm or you don't. I probably didn't explain that very well but they're just two very different things IMO.
I have a friend who had A cups and I know they were a cause of embarrassment for her for her entire life, and she got implants a couple of years ago. I do feel for her that she felt so self-conscious about it but I think most of us could look in the mirror and pick out something we don't like about ourselves. It's not on anyone else to fix those things for us...we either find a way to accept it, move past it, or if we're really stuck on it...save up money to get cosmetic surgery. As others have said, there are far more important things to spend money on than breast implants. And I don't see how THAT is subjective.
Actually I do agree with this, I think it's a good point. There are definitely differences; but implants in that case are still a form of prosthesis, by definition.
I guess if you are utterly fixated on an aspect of your body which you are missing (in a medical sense, not necessarily just that they are smaller than you'd like) then I imagine it would consume you and pervade all areas of your life. I guess that's where subjectivity is involved. If you have a medical condition that is ruining your life, it would seem pretty important to you.
It does seem hypocritical to me that people would support the NHS in providing implants to mastectomy patients but not to people who have no breast tissue.