Introduction to Air Law for Hobby Drone Flying
Small unmanned aircraft (Drone) Sub 20kg
Article 94 (Simplified)
(3) The person in charge of a small unmanned aircraft must maintain direct, unaided visual contact with the aircraft sufficient to monitor its flight path in relation to other aircraft, persons, vehicles, vessels and structures for the purpose of avoiding collisions.
(5) The person in charge of a small unmanned aircraft must not fly the aircraft for the purposes of commercial operations except in accordance with a permission granted by the CAA.
As the Remote Pilot (RM) of your Drone you must be satisfied that the drone is safe to fly.  If you are not satisfied you do not fly.
Article 95 (simplified)
You Must Not fly your drone
(2) (a)over or within 150 metres of any congested area;
(b)over or within 150 metres of an organised open-air assembly of more than 1,000 persons;
(c)within 50 metres of any vessel, vehicle or structure which is not under the control of the person in charge of the aircraft; or
(d)subject to paragraphs (3) and (4), within 50 metres of any person.
(3) Subject to paragraph (4), during take-off or landing, a small unmanned surveillance aircraft must not be flown within 30 metres of any person.
With a Permission for commercial operations (PfCO) these distances may be reduced with the correct planning and allow you to gain money for flying your drone.
These distances do not apply to the remote pilot or crew involved if briefed by the pilot correctly

Summary

Keep your drone within sight, (visual line of sight VLOS)

Do not fly higher than 400ft height (122meters)

Do not fly further than 500meters distance (1640.42ft)

Do not fly within 150meters of a built up areas or 150m of organised crowds over 1000 people.

Do not fly within 50meters of buildings, ships, cars, people.  During Take-off and Landing you may reduce to 30m from people out of your control.  (only with a PfCO you may reduce to 30m from buildings, vehicles and vessels.

Do not fly within 2 or 2.5nm of an airport (FRZ) and not within 1km by 5km from the threshold of the runway (REZ), doing so may result in criminal convictions. 

Flying with a Permission (PfCO)

A Permission for Commercial Operations (PfCO) allows the drone pilot, (remote pilot, RM) to fly for commercial gain.  This Permission is shown below and allows the Remote Pilot to fly with specific limitations and conditions that are set out below.