The UK Manual of Air Traffic Services Part 1 staes with regard to fuel dumping that
a) over the sea if at all possible: or
b) above 10000ft agl.
Exceptionally, if a) or b) is impracticable or inconsistent with safety, fuel may be jettisoned above 7000ft agl in winter and 4000ft agl in summer. For fuel to be jettisoned below these levels the situation must be unavoidable.
A vertical separation of at least 1000ft between aircraft should be maintained.
H365
ps Thanks for all the info so far.