It is a speed brake, manually deployed by the pilot, at will, and it is available to be used at any speed. It is usually selected some 100 - 200' above threshold before touchdown. It is not a lift spoiler, and it is not generally used in normal flight.
Lift spoilers are in the upper surface of the wing, 2 of which deploy automatically when weight on wheels logic detects contact with terra firma, and a further 6 are deployed by the pilot after landing.
And before you ask, no, there is no reverse thrust, just the aerodynamic bits I've described to assist the wheel brakes, to achieve the short landing performance which the BAe146 is known for.
I hope this helps.