This rule is in addition to VFR and IFR altitudes
quoted by gAMbl3. Does it mean that it is illegal to fly over a congested area at 1500ft AGL if one can't glide to a safe landing area away from people/property?
Rules of the air
3.1.2 Minimum safe heights
Except when necessary for take-off or landing, or except by permission from
the Director General, aircraft shall not be flown over the congested areas of
cities, towns or settlements or over an open-air assembly of persons, unless
at such a height as will permit, in the event of an emergency arising, a landing
to be made without undue hazard to persons or property on the surface.