I’ve not been able to find any legal prohibition specifically on overflying prisons with drones. There are several such regulations that apply only to helicopters.
The 164ft figure quoted is curious, and it is intended to be 50m clearance.
This page from Altitude Angel explains the basis of the 164ft “no-drone” zones above prisons included in their data:
You’ll note that it is just the 50m clearance to buildings contained in the old regulations. So despite what a few people have said above, there appear to be no NFZs specific to prisons that affect drones, and the data provided by AA seems not to be a formal NFZ of any kind.
So:
- you can overfly a prison entirely legally with a Mini or Mini 2 (from the pure CAA airspace point-of-view)
- you probably shouldn’t overfly at any height with any other drone (50m horizontal clearance likely to be required in Open Category A2)
- The Altitude Angel data is out-of-date and should really be updated
- I still wouldn’t go anywhere near the place with a drone of mine!