Regarding ATC guidance CAP493 says the following:
Fuel Jettisoning
1.77
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.
1.78
When an aircraft in controlled airspace needs to dump fuel, ATC should
co-ordinate with the flight crew:
1.
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;
2.
the level to be used;
3.
the estimated duration of the fuel dumping; and
4.
the frequency to be monitored whilst the aircraft is dumping fuel.
1.79
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.
1.80
A vertical separation of at least 1000 feet between aircraft should be
maintained.
1.81
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)
http://www.caa.co.uk/docs/33/CAP%204...dition%205.pdf