comes from the south west of London around the Wimbledon area... meaning a person is crazy or sick in the head to do such and obscene thing

Someone people turn to when they're sad who makes them feel better.

When someone does something cool or crazy they are a sickhead.

Someone who is Sick (in the) Head.
Hence Sick Head.
This is a person who tends to do crazy dangerous and slightly harsh things. This involves getting in fights and going crazy.
(See Jack Brett)

sick head can mean some one is sick in the head, but also can mean something is good.

someone who is really good at something

somebody who is sick at something. a sickhead could be an extremely violent person, or a stunt performer, a joker of all levels. the jack in the pack, the sickhead of the group will be popular providing he continues to be a sickhead.