Aircraft are subject to strict altitude and proximity limits which will affect how you plan your aerial shoot.
The full rules are available from the CAA Air Navigation Order - Rules of the air regulations, Section 2, Page 287 "Low flying rules"
CAA UK regulations state that an aircraft operating on a standard licence must not be closer than 500ft to a person, vehicle, vessel or structure. A filming exemption issued by the CAA can reduce this limit to 200ft and sometimes less.
Single engine helicopters: CAA UK regulations require the aircraft to have a glide path to a safe landing area if the engine fails. In addition, over a built up area, aircraft must maintain a minimum height of 1,000ft above the highest fixed obstacle within a horizontal radius of 600m. So a single engined helicopter can only operate at the minimum of 1000ft over a built up area if it is close to the edge of the area and there is a large open space suitable for a forced landing (not a crowded park) very near by and within its' glide slope. The height of operation will have to be increased to maintain the ability to glide clear as the aircraft moves further away from the safe landing area.
Twin engine helicopters: With a filming exemption, a twin engined helicopter can operate at a minimum of 750ft over a built up area regardless of the distance to the nearest safe landing area. This is because the aircraft can still fly on one engine if the other engine fails.
It is possible to fly outside of these limits but a special exemption will be needed from the CAA. For example, you can get a 20 metre exemption which allows you to fly very close to vessel or structure and as close as 20 metres from a person. Your aerial photography supplier should be able to advise you on this and you'll have to justify your reasons and explain your safety procedures to the Flight Operations Inspector at the CAA. You may also need to inform local authorities and the police so give yourself plenty of time and plan ahead as these permissions can take some time to put in place.