You are not allowed to shot aircraft that fly over. IIRC there
is a 250ft minimum altidude on aircaft as well, so that is
esentially where 'your property' stops and 'free airspace'
begins, for practical puroses.

There isn't. There is a 500ft rule in a direct line from any structure rule.
So if you had a farm and were away from any buildings, people, vehicles etc,
the trespass only starts when the wheels touch the ground. However, for a
standard house plot, there is effectively a 500ft minimum fly over height.
Over a congested area (which originally intended to mean towns and cities,
but is now harshly interpreted by the courts to basically mean anything from
a small hamlet upwards), there is a minimum 1500ft height.

However, the 500ft rule is not applied when aircraft are using the space to
take off or land. The 1500ft rule doesn't apply either, provided it is a
licenced aerodrome, which last time I checked the AIP, anything BAA operated
was, not surprisingly!

Christian.