Comes from The Lenny Henry Show from the late 80's in the UK. Lenny Henry's character (Delbert Wilkins) would often tell his friend Winston to calm down in his character's vaguely West Indian accent - "chill, Winston". The line used in Lock Stock & Two Smoking Barrels is a reference to this. It has become a common British saying, though many are unaware of it's origin.
Chill Winston! I've got it under control.