400ft
"a careless drone operator otherwise obeying the letter of the law shouldn’t be flying above 400ft"
Whereas a careful drone operator who knows the letter of law and is flying under CAP658 knows that there is no such limit, unless the aircraft weighs more than 7kg:
"4.2.2 Article 166 – Small unmanned aircraft
‘(1) A person must not cause or permit any article or animal (whether or not attached
to a parachute) to be dropped from a small unmanned aircraft so as to endanger
persons or property.
(2) The person in charge of a small unmanned aircraft may only fly the aircraft if
reasonably satisfied that the flight can safely be made.
(3) The person in charge of a small unmanned aircraft must maintain direct, unaided
visual contact with the aircraft sufficient to monitor its flight path in relation to
other aircraft, persons, vehicles, vessels and structures for the purpose of
avoiding collisions.
(4) The person in charge of a small unmanned aircraft which has a mass of more than
7 kg excluding its fuel but including any articles or equipment installed in or
attached to the aircraft at the commencement of its flight, must not fly the aircraft:
(a) in Class A, C, D or E airspace unless the permission of the appropriate air traffic
control unit has been obtained;
(b) within an aerodrome traffic zone during the notified hours of watch of the air
traffic control unit (if any) at that aerodrome unless the permission of any such
air traffic control unit has been obtained; or
(c) at a height of more than 400 feet above the surface unless it is flying in
airspace described in sub-paragraph (a) or (b) and in accordance with the
requirements for that airspace."
Also, when flying under the FPV exemption, the max altitude is 1000ft:
"4) The person in charge must not fly the SUA:
a) in Class A, C, D or E airspace unless permission of the appropriate air traffic control
unit has been obtained;
b) within an aerodrome traffic zone during the notified hours of watch of the air traffic
control unit (if any) at that aerodrome unless permission of any such air traffic control
unit has been obtained;
c) at a height of more than 1,000 feet above the surface (see Note 3);"