So the whole cannula / intubation thing then is incorrect. You don't need to do that to sedate someone.
For the sake of clarity, in a general sense, you do not need a cannula or intubation to give “sedation”. In the same way that you do not need to go onto cardiopulmonary bypass to have an “operation”.
Sedation is a spectrum of altered consciousness and the drugs, monitoring and interventions required to make it safe depend on a combination of the patient’s own risk factors, the level of sedation required and the reason for the sedation (including the likely duration, how easy it would be to stop halfway through or convert to GA etc).
This scenario is very different to elective dental surgery in a cooperative patient without risk factors.
If you need to have a procedure, and sedation is being considered, whether you are pregnant or not, you should discuss this directly with the person who would be giving the sedation so that you can give informed consent to it.
And if a woman not detained under the MHA or judged to lack capacity, for whatever reason decides on a course of non-intervention that might allow both the fœtus's death and her own, it is her right to do so. Isn't it?
Yes - the example of JW was given above, where it’s legal for people to bleed to death rather than accept transfusion.
But capacity is important, as people say all sorts of things in the heat of the moment / without full information etc etc and if everyone who ever used the phrase “I’d rather die than x or y” was taken at their word without any exploration that would seem a little remiss.