I don't know your situation but I get the impression that they are pretty strict in who they will consider. I'm also not too clear on whether people get all their money back if they are rejected. I strongly suggest researching all of that.
If it were an option for me though, I'd certainly consider it. I'd much prefer a guaranteed and hopefully peaceful death.
The other issues though would be finding someone to accompany me. I believe they require this to identify you before and after- as it were. That's a big ask of someone. Plus, the UK is going all 'nanny state' over this sort of thing at the moment. I wouldn't want to put them at risk of being arrested (for assisting a suicide.)
Plus, I feel like- if I were doing it officially, I probably would want to let friends and family know- which would be a tricky discussion.
I'm not vastly knowledgeable on it but I get the sense that Dignitas is all age 50+, chronic or terminal illness. Whereas Pegasos might be more open to considering things like long-term mental illness etc.