I don't think most people here are against OP having the surgery - I think most are against the idea of the NHS paying for it.
The reality is there are life saving cancer treatments for children that the NHS does not fund because they're too expensive. There are life prolonging drugs that similarly the NHS does not fund, because they have to prioritise conditions and who gets what.
The reality is that in order to pay for these extras, they do cut back on life saving treatment. I saw one example of a drug that would give children six months longer to live, and they said no because they wanted to fund something else.
There's mothers with psychosis who don't get proper treatment on the NHS (I know of someone this happened to, and their children's lives became in danger) because mental health funding in the UK is so depleted.
These are hard decisions, and there's just not enough funding for all of it. If the NHS funds OP's boob job, the money comes from somewhere - likely an emergency situation, because that seems to be the only thing the NHS is funding properly at the moment.
That's why people are arguing against OP - not because they don't think it's important she gets a boob job to help her mental health, or a good thing, but because there are more needy people who need that healthcare.