Tips
Turn signals:
In free roam mode, for added realism you can use your indicators (turn signals). Press L1 to indicate right and L2 to indicate left.

Gas tank kills:

Shoot a gas tank when an enemy is nearby to easily kill them.

Health recovery:
If you have been getting damaged badly by enemies and are close to dying, walk up next to a wall. Your character will lean up against it in order to recover. With each breath your character takes, part of his bullet holes disappear until he is fully recovered. This may take a minute -- make sure that no enemies are attacking you.

Hidden vehicles:

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.
DC Carter moves:
Knock out hostage: With a gun, press X to get a hostage then press Square.
Kick hostage: While unarmed, X to get a hostage then press Square.

Mark Hammond moves:
Shoot hostage: With a gun, press X to get a hostage, then press Square.
Break hostage's neck: While unarmed, press X to get a hostage, then press Square