As far as I know an ADRT has no legal requirement to "be accepted", past being witnessed by someone. The purpose of being witnessed by someone is to verify your mental capacity to make such decisions. The text of the act makes no reference to having the ADRT "accepted", and it also doesn't exclude using an ADRT to tell medical staff to do nothing to save your life. Specifically, Sections 24 through 26 deal with ADRTs, with section 25 being the "Validity and applicability of advance decisions". [Edit: "P" in this extract is the person the ADRT is written by and applies to]:
(4)An advance decision is not applicable to the treatment in question if—
(a)that treatment is not the treatment specified in the advance decision,
(b)any circumstances specified in the advance decision are absent, or
(c)there are reasonable grounds for believing that circumstances exist which P did not anticipate at the time of the advance decision and which would have affected his decision had he anticipated them.
(5)An advance decision is not applicable to life-sustaining treatment unless—
(a)the decision is verified by a statement by P to the effect that it is to apply to that treatment even if life is at risk, and
(b)the decision and statement comply with subsection (6).
(6)A decision or statement complies with this subsection only if—
(a)it is in writing,
(b)it is signed by P or by another person in P's presence and by P's direction,
(c)the signature is made or acknowledged by P in the presence of a witness, and
(d)the witness signs it, or acknowledges his signature, in P's presence.
(7)The existence of any lasting power of attorney other than one of a description mentioned in subsection (2)(b) does not prevent the advance decision from being regarded as valid and applicable.
My interpretation therefore is that, as long as you specify expicitly what you want to receive treatment for and what you don't, an ADRT can be used to stop medical staff or caregivers from intervening on a suicide attempt [Edit: or indeed anything else]. Certainly mine was filed with my GP without comment.
If you want to read the full text of the act, you can
find it here. Again, not a lawyer myself, do your own research etc. etc.