This is from Macmillan's website
"Macmillan Cancer Support is here to do whatever it takes to support people living with cancer."
Now I think taking the strictest reading of these words. I and your dad have been discharged, so we are not 'living with cancer' so by that definition, no. However if I and your dad are still living with the effects of cancer, a broader reading says, yes you can still contact them.
If I needed their services I would contact them, I wouldn't be worrying about the exact definition of who can or can't use their services.