You can make a Lasting Power of Attorney for Personal Welfare which covers health and welfare issues.  If you choose to do that the Lasting Power of Attorney has to be registered at the Court of Protection to whom a fee of £150 is due.  An alternative is for you to sign a Living Will.  A Living Will can record your wishes in respect of certain medical procedures that are/are not to be employed on loss of mental capacity (advance directive).  It can also record your beliefs on such procedures for the family to consider should a difficult decision with regard to your health have to be made (non advance directive).  A Living Will is therefore much cheaper than a Lasting Power of Attorney but gives to your family and the Doctors treating you explicit instructions on what they should do in the event of you being unable to communicate your wishes.