Gillick Competence is not to be confused with the Fraser Guidelines which is only concerned with contraception.
Gillick Competence is:
a term originating in England and is used in medical law to decide whether a child (under 16 years of age) is able to consent to his or her own medical treatment, without the need for parental permission or knowledge.
The standard is based on the 1985 decision of the House of Lords in Gillick v West Norfolk and Wisbech Area Health Authority. (Source: Wikipedia)
I am wondering what limitations there are to the ability for a child to override a parent/guardian's decision concerning their health and wellbeing.
Wikipedia at the same source, points out that (emphasis mine)
except in situations that are regulated otherwise by law, the legal right to make a decision on any particular matter concerning the child shifts from the parent to the child when the child reaches sufficient maturity to be capable of making up his or her own mind on the matter requiring decision.
What situations are there that are "regulated otherwise by law"?