"Provocation" is not a defence in English law (except as a partial defence to a charge of murder, where is has the effect of reducing the charge to one of manslaughter). You cannot rely on provocation as a defence to an assault charge.

You can rely on "self-defence" - but this has to be proportionate. For example, if your dad slaps you once, and you fear that he will immediately slap you again, then you can use force to protect yourself from being slapped again. You cannot intentionally "put him in the hospital" unless the threat he poses calls for that kind of force - to me, this says that you intend to use disproportionate force.

People get this funny idea about the use of force, that there's this special, Home Office-approved 'List of Injuries You're Allowed to Inflict in Self-Defence (3rd Edition, illustrated)'. There isn't. Your force must simply be reasonable - so that a reasonable person would say, "in the circumstances as he believed them to be, that level of force was reasonable".