I'm sure there are unhealthy things you could do to increase the risk of illness. Nothing is guaranteed though. Plus, there's no control over how long it will take you to die. So, then it might come down to how long you can sit tight with excruciating pain- possible without a diagnosis or painkillers- if you want to avoid medical intervention.
As an example, one morning years ago, I got sudden terrific clamping pain in my chest. Panic started to set in, slight difficulty breathing. I thought- this is it. This is a heart attack! Ok- you're ready to die, don't phone for an ambulance. The next maybe 20 minutes, I didn't know what to do with myself. Tried lying down. That made it worse. Literally crawled to the window for some fresh air. Eventually, it passed but, I didn't (obviously!)
Between NHS incompetence and my lazyness, I had 13 more attacks until the pain wasn't fully easing up after an attack. After a few days, I couldn't stand it anymore and felt like I had to get it taken care of. It was a gallstone and a bunch of sludge in the bile duct. I maybe would have died eventually if it had gone untreated. I don't know how long it would have taken but, I know I couldn't outlast the pain. I just wanted it gone. So, unfortunately, I don't think it's necessarily as simple as- contract something nasty and, die quickly. Maybe it is for some but, I don't know how you could control the process.
I'm also not sure how doctors would respond if we go for diagnosis but then, reject treatment. It's our right- of course. But, I wonder how eager or even obliged they are to then offer pain management. It's something I've always wondered about actually.
Also in terms of assisted suicide- if it ever gets brought in. If you refuse medical treatmemt, will a doctor then support an application for assisted suicide? I have a horrible feeling that they wouldn't.