In the England and Wales ones right to respond to attacks is limited to what is considered "reasonable force". The CPS sum up with a quote from Palmer v R,  AC 814:
It is both good law and good sense that a man who is attacked may defend himself. It is both good law and good sense that he may do, but only do, what is reasonably necessary.
In the case of a fist fight, this is sometimes expressed as if someone punches you, you are allowed to punch them but not allowed to pick up a bottle and hit them with it. However, many fights end up in a grapple on the ground (estimates vary from 42% to 95%). Traditional ways to end a grapple involve causing cerebral hypoxia (chokeholds and strangles) or dislocation (joint locks). These have potentially life changing or life ending consequences, particularly when a submission is impractical. Other techniques such as biting and eye gouging are less likely to be fatal but more likely to cause life changing injuries.
How have court cases interpreted reasonable force in cases where an attacker suffered serious injuries as a result of a fight they initiated ending in a grapple?