If someone ends up in a state where they can no longer care for themselves or express themselves, I imagine a lot of decision making falls on the parents or family/next of kin.
Leaving a note behind relies on whoever finds the note honouring your instructions. If it's your loved ones and you already doubt they would support you in the right to die, I'm not sure, it's kind of unknown.
I guess you could send a delayed email to someone official like a solicitor. But yeah- speaking to one beforehand would make it obvious what you were about to do.
I really don't know if people are obliged to follow such instructions though. Obviously, there are things like 'Do Not Resuscitate' orders. You'd really hope that our family members would honour our wishes but I'm not sure they are legally obliged to.
Plus, there's the whole mental capacity card. Will they simply say that wanting suicide is a symptom of mental illness so- that person isn't in a position to make decisions like that? Obviously, I disagree with that but- I'm sure they would use that.
Do you really think your parents or loved ones would keep you alive- no matter what your condition?