Caveat: I have not been in any kind of physical altercation since 2007. I am, however, extremely familiar with violence. I hold instructor ranks in three separate but very similar martial arts, Ninjutsu, Savate, and Muay Thai. I can't say anything about the UK or their laws, but here's my take on my experiences with the US law.
As it has been explained to me by a few lawyers, a bunch of police officers, and all of my coaches/teachers: it's not proportional force that law enforcement is looking at in cases of self defense, it's escalation of force. If I put down a violent threat, end the fight, and then immediately call the cops (and wait around to give a statement,) without going out of my way to cause extra damage to my assailant, they're not going to prosecute me.
I used to barfight frequently. I have always been told that the "appropriate measure of force," was "enough to stop someone from hurting you or others and no more." I have been in a couple of fights where a guy just refused to stop getting up until I absolutely wrecked his shit, and have never even seen the inside of a jail.
If, hypothetically, I were to kick someone, then jump on them and ground and pound them after they stopped fighting back, then I would definitely be a candidate for jail, deservedly so. Especially if I were to GnP after they were unable to defend themselves, like unconscious.