I merely agree that some issues have no convenient or close analogy.
Don't agree.
A consenting person requesting that a life support machine is turned off seems almost entirely analogous to assisted dying. Both involve the consent of an individual who wishes to die but cannot do so without the assistance of others. In both cases the death is as a direct result of the intervention and in both cases the assistance is required as the individual themselves is not in a position to take direction action themselves to allow them to die - they can only do so with assistance.
There are differences, however when we look at the details of the Bill going through parliament and what is already permitted under law.
The turning off of life support machine requires a direct intervention by a third party to ensure that the person dies - in other words someone other than the patient will turn off the machine. For assisted dying, although there is assistance to prescribe the medication, only the patient themselves would be permitted to take them - they cannot be administered by a third party.
The current law allows individuals to make advance directives allowing turning off of a life support machine under circumstances where they can no longer consent. That isn't the case for the proposed assisted dying - where the person must be competent both when they request and when they take the drugs.
The current law allows others to take decisions to turn off life support on behalf of a person who is not competent. The proposed assisted dying does not allow anyone other than a competent patient to take those decisions.
The current law on switching off of life support only required high court authorisation under 'difficult' cases - where a competent person is consenting for themselves they would not get involved. The proposed assisted dying bill requires high court authorisation in all cases, even when an analogous case (competent person consenting) would not require this for cessation of life support.
So while the fundamentals are pretty well identical - person wishes to die and cannot do so without direct/indirect third party intervention - the proposals are way more strict on assisted dying than cessation of life support.