When you have been shot, and you badly need health walk up to a wall (any) and stand next to it close he will then lean up against it, it may take a few seconds but wait till he gets off, he will have a full recovery.
Press Up(2), Left(2), Right(2), Circle(2), Down at the title screen. The sound of a woman groaning will confirm correct code entry.
Infinate Ammo W/ No Reload
Enter this code during the video at the start of the game up, down, left, right, triangle, up, down, left, right, square. You should hear the sound of a woman screaming if entered correctly.
Armored car weapon:
Press Up, Down, Left, Right, Square, Triangle, Circle at the title screen. The sound of a woman groaning will confirm correct code entry. Note: The weapon can only be used during missions.
Skip intermission sequences:
Press R3 to skip some intermission sequences. Note: Not all sequences can be skipped.
Free Roam mode:
Successfully complete all standard missions to unlock the "Free Roam" option.
Kelvin Deamer mode:
Successfully complete the game to unlock free roaming mode. Then, go to the golf cart that you will find in one of the parks. Press Up, Down, Triangle, Down, Triangle, Left, Right, Circle, to be transformed into a character known as Kelvin Deamer.
You can find some hidden vehicles in the parks of London, such as a sooty van (Little Suzuki van with grass in the back) and a very fun golf cart parked behind huts and bushes around the parks.
In free roam mode, for added realism you can use your indicators (turn signals). Press L1 to indicate right and L2 to indicate left