It literally is a fallacy. This is not up for debate. The slippery slope fallacy is a real fallacy, and this is an example of that fallacy.

And again, this is nothing like the protocols Canada has.

You need to be terminally ill with less than 6 months to live, of sound mind, have the go-ahead from two unaffiliated doctors, and it needs to be reviewed and signed off by a judge.

You're advocating for real, horrific, suffering to continue because hypothetically the law could be changed in future in a way that could be bad.

I've worked in care homes full of people who barely sleep, and spend their entire days in agony that you and I cannot even conceive of. They begged to die. They begged us to covertly kill them. But our job was to forcefully keep them alive against their will, prolonging their suffering for as long as we possibly could. No attention given to their comfort or quality of life, just ensuring they are kept alive as long as possible. That's what we had targets for. Seriously harrowing stuff.

If you had seen that, day in day out, I doubt you'd have this "we need to make them suffer, because hypothetically in X years we could be like Canada, where some doctors made a recommendation they really shouldn't have."

Regardless, it's pointless talking about. Your viewpoint has been rejected by the populace and most importantly, by MPs.