500' Ceiling?...
It's a bit more complicated than that. In military terms, jet 'low flying' is anything below 2000'. In general though, there's not much training advantage to flying that high, so over most of the UK, IF the jet is at low level, it is likely to be (aiming to) cruise at 250' above ground level (AGL). In certain defined areas, crews may be cleared down to 100' AGL - the highlands being one such area. Rotary aircraft will be aiming to fly around 50', but are effectively cleared down to the surface (ie, hovering, simulating dropping off passengers, and to fly under power lines).
As to the rules/precautions, it is up to the crews of civil and military aircraft alike, to keep and good lookout and avoid each other. Aids may be available to do this (RADAR, Air Traffic advice and reports), but in the valleys this isn't available - so you are relying on
a) mark 1 eyeball, and
b) the Big Sky theory.