I've recently submitted a PIP claim myself. I spoke to MacMillan and they asked me if I wanted to claim under the Special Rules for terminal illness. Pretty sure my nurse specialist would have signed as DS1500, which is the form you need for a terminal illness claim but I made a very conscious decision not to.
I have lung, liver, bone and brain mets so not great but no one has told me it's terminal. If it is "terminal", my options for travelling are massively limited as insurance becomes prohibitively expensive. Currently my employer pays for my insurance as I have work travel as well and the policy covers personal travel too. The insurer defines terminal as less than a year to live. Just something to consider but this was a big deal for me.
Getting a DS1500 completed seems to be down to the person being asked to sign it. I know people who are really ill who haven't been able to get this signed and others who are doing really well who have.