Found the info below on a website: http://www.protectingyourself.co.uk/self-defence-law.html
This comports with my understanding on the law in the US and the UK. Please note that all actions with or without weapons are judged by the standard of reasonbleness. If you pummel an unarmed person half your size or an an unarmed woman with a roll or quarters because they took a swing at you, it could be a problem depending on the threat they posed to you. On the other hand if the guy is a rugby player who is larger than you, using a roll of quarters might be justified even if he is unarmed. If someone smaller has a knife, I do not think a judge or jury will have any problem with the roll of quarters scenario. One of the best brawls I was ever in, a buddy next to me who weighed about 130 lbs knocked out a monster with one punch. Turns out he was a golden gloves boxing champ which we did not know till that night.
Here is what the website says:
There has been confusion about what is permitted under the law when an individual is acting in self-defence. Some have even suggested that the law gives more protection to criminals than to honest citizens acting to protect themselves, their family and their homes. There is a belief that citizens in the USA are in a much stronger position as far as the law on self-defence is concerned.
However, although not enshrined in statute, the law in this country is very clear:
an individual is entitled to protect themselves or others;
they may inflict violence and/or use weapons to do so;
the level of violence may include killing the assailant; and,
an individual may even act pre-emptively and still be found to have acted in self-defence.
The protection offered to the honest citizen by the principle of self-defence comes in two stages.
The Crown Prosecution Service
Before a case gets to court the Crown Prosecution Service (CPS) will have to decide whether it should go that far. In reaching this decision there are various factors that the CPS will take into account, including:
Whether there is likely to be enough evidence to secure a conviction; and,
Whether a prosecution is in the public interest.
The CPS has stated that citizens who have acted reasonably and in good faith to protect themselves, their families or their property should not face prosecution for their acts.
There will be instances where the circumstances of an individual case demand that it goes to court. These may include cases where it is not clear that an individual really was acting in self-defence or where serious injuries or death have resulted. However, this does not mean that a death will automatically lead to prosecution.
Self-Defence and the Courts
If an individual is prosecuted after having acted, or having claimed to act, in self-defence the courts will apply the following test:
Was the force used by the individual reasonable in the circumstances as he or she believed them to be?
The jury will have to answer this question based on the facts as the individual saw them when he acted as he did. A person is entitled to use reasonable force to protect themselves, members of their family or even a complete stranger if they genuinely believe that they are in danger or are the victim of an unlawful act, such as an assault. An individual may even take what is known as a pre-emptive strike if they honestly believe that the circumstances demand it. This means that a person can use force if they believe that there is a threat of imminent violence if they do not act first.