Current low flying rule is in SERA, however, see Civil Aviation Authority
SAFETY NOTICE Number: SN–2014/010 Appendix a which modifies SERA. Extract below.
Part A Exceptions to the Minimum Height Requirements
1) General (SERA.5005(f)(2))
a) The CAA permits, under paragraphs SERA.3105 and SERA.5005(f), an aircraft to fly at a height of less than 150 metres (500 feet) above the highest obstacle within a radius of 150 metres (500 feet) from the aircraft, subject to the condition set out in subparagraph (b).
b) The aircraft must not be flown closer than 150 metres (500 feet) to any person, vessel, vehicle or structure except with the permission of the CAA.
2) Landing and Taking Off
The CAA permits, under paragraphs SERA.3105, SERA.5005(f) and SERA.5015(b), an aircraft to fly below the heights specified in SERA.5005(f) and SERA.5015(b) if it is flying in accordance with normal aviation practices and:
a) practising approaches to land at or checking navigational aids or procedures at a Government or licensed aerodrome;
b) practising forced landings if it is not flown closer than 150 metres (500 feet) to any person, vessel, vehicle or structure; or
c) flying in accordance with a notified procedure or when specifically authorised by the CAA in accordance with SERA.5015(b).