The aircraft must be kept within the visual line of sight (normally taken to be within 500 m horizontally and 400 ft vertically) of its remote pilot (i.e. the ‘person in charge’ of it). Operations beyond these distances must be approved by the CAA (the basic premise being for the operator to prove that he/she can do this safely).
Small unmanned aircraft (irrespective of their mass) that are being used for surveillance purposes are subject to tighter restrictions with regard to the minimum distances that you can fly near people or properties that are not under your control. If you wish to fly within these minima, permission is required from the CAA before operations are commenced.
CAA permission is also required for all flights that are being conducted for aerial work (i.e. in very simple terms, you are getting paid for doing it).
The 'remote pilot' has the responsibility for satisfying him/herself that the flight can be conducted safely.
The aircraft must not be flown: