ralphmalph in IACO its 500' agl as well as all the rest of rule 5.
The UK doesn't have the 500' agl but has a 500' bubble on "any person, vessel, vehicle or structure"
You can fly down Lochs at 2" agl if you like and be perfectly legal.
Most of the FW light aircraft below 500ft will be doing PFL training. The CAA have for years tryed to stop instructors doing this but alot of us still do because if you knock it off before 500ft agl you miss a large part of the difficult bit and duty of care and all that good stuff.
Then there is trial flights where the punter wants a photo so 500ft agl over thier houses.
But to be honest alot of instructors now have been trained abroad so don't have that much clue about mil low level flying.
The fact that the MIL AIP has been made a revenue generator means most don't have access to the required information to be able to not do stuff in silly places. Doesn't help either.