Check out UK AIP ENR 1.2-1 para 1.3
(a) General
(i) The Civil Aviation Authority (CAA) permits, under paragraphs SERA.3105 and SERA.5005(f), an aircraft to fly at a
height of less than 150 m (500 ft) above the highest obstacle within a radius of 150 m (500 ft) from the aircraft,
subject to the condition set out in sub-paragraph (a)(ii).
(ii) The aircraft must not be flown closer than 150 m (500 ft) to any person, vessel, vehicle or structure except with the
permission of the CAA.