Could this be prosecuted as FGM? Breasts are a reproductive organ and they are facilitating mutilation.
Interesting question. There's some regions in Africa (forgot where) where they iron girls' breasts to stop growth so that the girls won't be molested by men too early. Sheer desperation, I guess, but if they then emigrate to the UK, there might be legislation about mutilating the breasts due to this "tradition".
(Anything misogynist, as we know, is only perceived as bad when foreigners do it)
You might get them with those laws.
Regular FGM, I don't think. Breasts are a secondary sex organ, not a primary one, there's probably a difference in legislation.
There's probably also more general laws about encouraging children to self-harm that might apply, but you'd have to get a judge to apply them to something that is to do with transgenderism, the holy cow of modern society.