doctors are just people, and as such sometimes get things wrong, just as your dad got things wrong when he "discharged himself" (by that i take it you mean it wasnt the hospital who discharged him, rather that he went against medical advice)
Sometimes diagnoses arent outwardly apparent, and unless there was negligence on the part of a hospital practitioner, you dont have much chance (for example if a hospital doctor knew that the ulcer was about to burst but didnt do anything to prevent it, but i dont see how people can see into the future)
Also, you mention about having the strength to sue, but how will suing make you feel any better, bring your dad back or in anyway help the nhs improve for other people(ie they will ultimately have less money to change things, or employ more cleaners, for example if they have to pay you money)
Wouldn't it be better to contact your local patient advice and liaison office (PALS) and make a formal complaint about what you thought was wrong with your dads care, then they can take it forward and investigate, and if there is anything that cn be changed for the better for future patients they will