Whereas your Online or Bespoke Will deals with your intentions after your death, a Living Will can deal with your medical requests whilst you are still alive.
A Living Will is not a really a will, it was the popular name for an Advance Directive, which was a way of explaining how you wished to be cared for in future if you lost mental capacity.
Since April 2007 the legal term became an Advance Decision, as covered by the Mental Capacity Act 2005.
An Advance Decision is a statement, that can either be written or oral, which sets out what medical treatment you wish, or do not wish, to undergo in specified circumstances before your death.
Advance Decisions can be very useful because they will be considered if you lose the power of making decisions or communication (e.g after suffering a stroke or falling into a coma). They can tell your doctor and relatives what you do and / or don't want done, without them worrying that they might not be doing what you wanted.
An Advance Decision can be orally revoked by you at any time.
If you want to allow somebody to make decisions for you if you become incapable of acting for yourself during your lifetime you will need to grant them a Power of Attorney