Daffy, that may not be quite so easy, you need to completely compus mentus to go down that track or Dignitas will not touch you, if you have started showing symptoms of Dementia it may well be too late.
Dignitas are extremely expensive, You could just take out a massive loan, don't worry to much about the repayments.
Personally, I would wait a while to see how it progresses, then deal with it as I feel fit, I certainly wouldn't be paying anybody. I do however support the work of Dignitas as a dignified way out.