No problem. Happy to help!
To your last points. Those holding a Police Air Operators licence (Rule 6(e)) are exempt from the Low Flying Rule.
As far as the air ambulances are concerned, if you are close to their pad, then they except when performing a landing or taking off manouever (Rule 6(aii)).
The 500 foot rule is the general catch all, but aircraft also have to observe a 1000 foot rule over congested areas and open air festivals. Essentially this is designed so that single engined aircraft can glide clear if the spinny thing stops spinning. CAA in the UK takes all this very seriously. Student pilots have to pass an air law exam with all of this stuff (and lots more) before they are allowed to go solo.