My idea is important because there have been numerous occasions where the police have prosecuted the victim of a crime merely for attempting to defend themselves. Not only this, people are afraid to defend themselves in case they get prosecuted, despite the likelihood of any action they need to take being legal. However, as mentioned above, it is down to the Police (and one assumes the CPS) to decide what is and is not 'reasonable force'. The Police service has a vested interest in preventing the public from defending themselves against harm when a crime is being committed, as it is seen as their role. Of course they want to hold on to their position as the 'protectors' of the public when in fact they cannot be omniscient and cannot be in all places at all times.
I believe that the current law on Self Defence is tilted in favour, not of the law abiding citizen, nor of the criminal but of the police. It is effectively down to the Police to decide what is and what is not reasonable force, and as such they use it to their advantage to cement their position as 'protectors' of life and property.
An adult human can make decisions themselves, and policepersons cannot be in all places at once. It would be preferable to set out a specific and unambiguous framework for self defence that leaves no possibility for the innocent being punished, nor the guilty being let off or able to litigate against the victims of their crime.
It should be legal to use force to defend oneself if an intruder enters ones property for example, and it should be legal to use force to defend oneself 'in the street' if you feel that your life or health is in danger. This would be similar to the US situation where a person is allowed to defend himself against an assailant.