UK CAA document CAP 493 Manual of Air Traffic Services Part 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 ATC.
12.2 When an aircraft in controlled airspace needs to dump fuel, ATC should co-ordinate with the flight crew:
a) the route to be flown which, if possible, should be clear of cities and towns, preferably over water and away from areas where thunderstorms have been reported or are expected;
b) the level to be used;
c) the estimated duration of the fuel dumping; and
d) the frequency to be monitored whilst the aircraft is dumping fuel.
12.3 Controllers are to recommend to flight crew that jettisoning of fuel should be carried out above 10,000 feet agl. Exceptionally, if fuel dumping at this level, or over water, 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.
12.5 Adjacent ATC units and control sectors should be informed of the fuel dumping taking place, including co-ordination with units providing services outside controlled airspace where the aircraft’s track is near to the boundary of controlled airspace (both laterally and vertically).
As usual the authorities have chosen to hide this information in their official publications available for free from their website. The swine!
As for max altitude, check your AFM limitations section. If nothing is stated then there is no limit.