There has been some debate recently as to whether the Air Navigation Order applies to drones flown indoors.  This extract from the CAA website clarifies the position:

 Indoor use – The applicability of the regulations regarding flights within buildings has been clarified recently.  Under the CAA Act 1982, the Air Navigation Order is made for the purposes of regulating air navigation.  Flights inside buildings have nothing to do with air navigation because they can have no effect on flights by aircraft in open air. As a result, flights within buildings, or within areas where there is no possibility for the unmanned aircraft to ‘escape’ into open air (such as a ‘closed’ netted structure) are not subject to air navigation legislation. Persons intending to operate drones indoors should refer to the appropriate Health and Safety At Work regulations. (sic).

It should be noted that this does not mean that no law applies.  The HSE are the regulatory body for drones when they are on the ground whether or not they are indoors or outdoors.  Indoors they are still a piece of work equipment and subject to the laws requiring risk assessment and suitable control measures for their safe use. Not least in a venue, operators need to be mindful of interference with such things as laser beams for smoke detection.

A stadium with no roof would constitute being outside because there is a possibility of a drone escaping into open air.

Drone technology is rapidly advancing and official guidance is still evolving so this is an area that needs to be regularly reviewed.