Last year, a 55 year old male grabbed me round the throat, pinned me against the wall and threw boiling water over me. I was badly burnt, I called the police and it went to court and social services were involved. During the video interview I was asked "was the boiling water in reach of the man, or did he go and get it purposely to throw over you?" I said he went to get it, and he got charged with the assult of a child. The moral of this is, you are allowed to use self defence, but cannot purposely carry weapons incase you get in a fight or get mugged. However, you can use what ever is around you and within reach to defend yourself. Therefore hitting them with your bike would be completely fine, whereas if you saw a bunch of chavs and reached into your bag to get out a spanner, that would not be allowed. You are allowed to use keys in self defence, and the police even recomend that. So if you tied a loop of elastic on your bunch of keys and started swinging that around you like a mad man, you could be safe and legal, providing you keep the keys in your pocket.