I think the classification of what types of treatment should or shouldn't be funded is really murky water and not something I've thought about enough to be able to describe 'should fund' and 'shouldn't fund' categories. I'm inclined to agree with you on that example, but if the rationale is the MH effect, that might also justify a lot of other surgery that I don't necessarily believe should be funded. So for example, someone who is a B cup and desperately unhappy about it might also say their MH is impacted, but would we still think the NHS should fund that surgery? My initial reaction is no. Equally I'm sure a trans person would argue that their surgery is 'corrective' so again it gets tricky.
Something else that has to be considered as well is how common the condition is. So if asymmetry to that extreme isn't very common, I don't see an issue funding correction. But then you get into drawing lines about how extreme it has to be e.g. would a B and a D warrant the same funding? Lots of women have breasts one or two sizes apart, should we start funding all correction? There are plenty of aspects of healthcare the NHS doesn't fund and generally the rationale is because it's so common it would cost too much - thinking here eye care, lots of dental care, etc. The number of trans children and adults seems to be increasing exponentially; surely there will come a point at which it is so common to be trans that trans related healthcare and surgery etc falls into this 'too common to pay for' category?
As I've mentioned before, women with PCOS aren't funded to have facial hair lasered off, so I'm quite uncomfortable with the idea that trans people might be obtaining procedures that biological women are not allowed because what - it's not deemed 'distressing enough' for bio women? I don't really have a conclusion on this so FWIW I'm not necessarily disagreeing with you, but what seems obvious to me is that these are conversations that really urgently need to be happening in relation to trans healthcare and I'm very concerned about the potential disparity - possibly discrimination? - when compared with biological women's healthcare