And, once again, kids can't get surgery in the uk.
They most certainly can.
Legal age for mastectomy on the NHS in the UK:
16 in Scotland
17 in England
18 in Wales and Northern Ireland
Obviously you have to find a surgeon willing to do this and I believe NHS surgeons in Scotland refuse to do them before 18, but it would certainly be legal.
Having internal and external sex organs removed and simulacra of opposite-sex genitals constructed is also an operation legal in underage children in Scotland and England at the same age as mastectomies, but again not done by NHS surgeons until the patient is 18.
There are no private surgeons advertising to underage patients that I could find at a quick search, but this doesn't mean they don't exist. I've certainly found patients claiming to have had mastectomies under 18.
Cross-sex hormones are sold privately and/or prescribed by private doctors and used by unknown numbers of UK children. We do know that the Webberlys prescribed them to children as young as 12. There is no law regulating this, the NHS however does not approve usage in the under 16s.
Now you may quibble about whether 16 and 17-year olds are still children but judging from my experience with kids that age, they very much are still children in many respects. As the brain doesn't mature until our mid-twenties, this isn't a surprise.