Looking at ORS4 No 1124, Paragraph 3b suggests we are allowed to fly a practice landing down to 'zero' ft. into a country field, as long as the farmer or his tractor are not in it....
b) practising approaches to forced landings other than at an aerodrome if it is not flown closer than 150 metres (500 feet) to any person, vessel, vehicle or structure; or
Just remembering my miles of low flying training at 30ft (altimeter) above the Dee Estuary.