Your next of kin (and other people close to you) do not have any legal authority to make decisions about your health and welfare if you lack capacity and are unable to communicate those decisions for yourself.
If you lose capacity and you have not made an Advance Decision or a Lasting Power of Attorney for Health and Welfare (LPA), then your doctor will decide what treatment to give you. They will base their decision on what they believe is in your best interests but this may not be what you would have wanted.
Your doctor should speak to your family or next of kin but does not legally have to follow what they say. This means that if you have no Advance Decision or LPA, the doctor has the final say about what treatment you receive. Your family can challenge their decision in the Court of Protection, but this can be costly and time consuming, and ultimately the final decision will then lie with the judge.
In your Advance Statement, you can write the names of your next of kin and anyone you would like to be consulted when decisions are being made on your behalf. However, this does not mean that a healthcare professional has to follow what that person says. The only way to give another person the legal power to make health or care decisions on your behalf is by making a Lasting Power of Attorney for Health and Welfare.