I can't say for sure about England,but it's probably the same as here in Canada.

If two guys square off for a scrap, It's called Consesual assault.
In other words its an assault that they have both consented to and Niether one can be charged by the other,or by the crown (governmnet)
If you go down the street and run and smash into some guy and drive him to the ground its an assault, If you do it in a football game (American football) its called a tackle. You cant charge a guy for tackling you in a game or body checking you in hockey or say punching you in a boxing match because both participants have consented or have "implied" consent.
The problem arises when one person goes beyond what the court deems the other has consented to.
As an example. In a street fight you punch a guy in the nose and break it and he is knocked down. The judge would say its reasonable for him to assume that this might happen in a fist fight.
If while he's down you punch him repeatedly in the face and cave his head in. It would now be an assault, or in extreme cases (kicking a guy who is down on teh ground) an aggrevated assault, which is a crime just a step below attempted murder and is VERY serious.
There are of course curcumstance where you can use force and even deadly force.
Self defense
Removing Tresspassers forcibly
Defending others
Hoilding a person awaiting police etc.
In each case only a "like" defensive force can be used and the cases are based on "reasonableness"
I worked as a bouncer in Edmonton in the 80's and have many bar and "street" fights as well.
I have had assault charges pressed against me 8 seperate times and have beat them every time. Ive had the charges dropped or "stayed"
In each case I immediatly counter charged and even after winning had my charges "held in abayance" which means I could press forward with them later.

I think the "registered deadly weapon" stuff is bull.
BUT if a guy was a pro boxer and got into a fight. (SAY TYSON) he could defend himself and knock the guy down or out. If he set the guy up and then hit him so hard he brain damaged the guy the consent and reasonableness rules would result in civil and probable criminal charges

If you are a tough or skilled guy and go around picking fights and pounding guys who are stupid or just drunk then you are a low life and deserve to go to jail.

P.S. I trained Karate for years but in the bar its hard to puch a guy and land clean,you end up breaking your hand on teh top of his head missing etc. What I ended up doing.(mind you Im 6'4" and was 270) I always would put my left hand around the back of teh guys neck and pull down and forward. They would always resist and pull their head straight up and back and then I would swing my right elbow around and drive it right into thier face with all my force and bodyweight behind it. I NEVER had a guy survive that.