To chip in some more - the "300 feet" reference mentioned a "built up area". The correct term for UK regs would therefore be in accordance with para 3(c), rather than the 500 foot rule:
SECTION 3
LOW FLYING RULE
Low flying prohibitions
5.—(1) Subject to paragraph (2), an aircraft shall comply with the low flying prohibitions in paragraph (3) unless exempted by rule 6.
(2) If an aircraft is flying in circumstances such that more than one of the low flying prohibitions apply, it shall fly at the greatest height required by any of the applicable prohibitions.
(3) The low flying prohibitions are as follows—
(a) Failure of power unit
An aircraft shall not be flown below such height as would enable it to make an
emergency landing without causing danger to persons or property on the surface in the event of a power unit failure.
(b) The 500 feet rule
Except with the written permission of the CAA, an aircraft shall not be flown closer than 500 feet to any person, vessel, vehicle or structure.
(c) The 1,000 feet rule
Except with the written permission of the CAA, an aircraft flying over a congested area of a city town or settlement shall not fly below a height of 1,000 feet above the highest fixed obstacle within a horizontal radius of 600 metres of the aircraft.
(d) The land clear rule
An aircraft flying over a congested area of a city, town or settlement shall not fly below such height as would permit the aircraft to land clear of the congested area in the event of a power unit failure.