From the UK MATS Pt 1......
12 Fuel Jettisoning
12.1 Pilots of aircraft in flight are permitted to jettison fuel in an emergency. The decision to jettison rests solely with the pilot but he may request guidance from air traffic control.
12.2 Controllers are to recommend that jettisoning of fuel should be carried out:
a) over the sea, if at all possible; or
b) above 10 000 feet agl.
12.3 Exceptionally, if a) or b) is operationally impracticable or inconsistent with safety, fuel may be jettisoned above 7000 feet agl in winter and above 4000 feet agl in summer.
For fuel to be jettisoned below these levels the situation must be unavoidable.
12.4 A vertical separation of at least 1000 feet between aircraft should be maintained.
WF.