The one thing that stood out to me OP, is what you said here:
"yes his diabetes was poorly controlled. He had been diabetic for around 30 years."
I am not medically trained and I do not have diabetes, but even I know that diabetes can kill if you don't control it.
I also know that diabetics should be checking their feet daily for signs of diabetic foot (especially if their diabetes is not well managed).
If your DF lived with badly controlled diabetes for 30 years, he will have no doubt seen HCPs with some regularity. All of whom would have told him that he needs to manage his diabetes better (sometimes with something as simple as a healthier diet and more exercise) AND that he should do daily feet checks.
Now I do not wish to take away from the tragedy of your father's death. However, your Dad lived with a medical issue that he knew could kill him if he didn't manage it properly and in your words he chose not to.
I'm sure you're in pain and you are looking for someone to blame, but the doctors will have told your Dad how to avoid getting diabetic foot, time and time again. How to check for it, to go to a HCP at the first signs of a foot infection.
You said that his foot had got to the stage where you wanted to throw up when you looked at it and at that point he went to A&E.
I do think individuals need to take some personal accountability for their health. In the same way that a chain smoker is told they have a high chance of getting lung cancer; people roll those dice with their health and think that the NHS will pick up the pieces and put them back together, but they can't always do that.
They no longer have the funding, the staff, the time or, in some cases, the will to help people; especially those who aren't helping themselves.
If you want to put a complaint in, then do so, but I think you need to admit to yourself that you're also angry at your Dad, for not looking after himself better, for not getting treatment for his foot sooner and for not taking precautions that would have led to him being around longer both for you and your mum.
I'm very sorry for your loss, it must really sting when it was potentially preventable.